I don't believe you. I don't believe that you don't have a single solitary story idea. Sorry.
SKIP AHEAD:
You have A Book Idea, But You Don't Think You Have The Writing Skill level To Pull It Off.
You'll never learn until you try. If you're scared that you'll kill your idea in the crib, trust me, that's going to happen anyway. You're going to have to write, re-write and edit your book until you can't remember what your own name is and aren't sure what character is and the word "said" becomes some strange four-letter word that makes your brain bleed. If you are going to spend your time, money, and brain power on something, make sure it's something you are interested in. Practice makes perfect is a great concept, but every book is different. They are all going to present new challenges. If you practice with writing prompts and the like, you'll be learning how to write a writing prompt and how to develop someone else's idea rather than learning how to write your idea. It's like picking up a guitar to learn to read music when you really want to play the piano.
Frequently I see the comparison of a new writer being like a toddler learning to draw or write. If you don't practice your messy scribblings on trees and noseless faces, then how will you ever grow up and be able to paint with oils? The comparison sounds apt, but you are not a toddler. You have your fine motor skills down pat. You've read books, written words, you've done all the messy scribbling, now it's time to pick up the oils.
You Think Your Idea Is Too Complicated
I have to admit, this haunts me at night. Chilling my bones and rattling chains in my non-existent basement. I wrote an extremely complicated story—perhaps too complicated, we will see. I fell prey to the idea that great stories are simple. Love, loss, magic. The basic stuff that can be distilled into two or three sentences and used in an elevator pitch.
Popular book Elevator pitches (that I made up and are not credited to the authors):
Lord Of The Rings — The noble quest of a hobbit and his buds to destroy a magic ring.
Pride and Prejudice — I have no idea. Enemies to lovers?
Twilight — ordinary teenage girl preyed on by a vampire, bafflingly a love story.
Hunger games — Gladiator but with children
Harry Potter — boy wizard who attends a magical school for witches and wizards.
Admittedly I haven't published (yet—chapter 8 of editing, woohoo!), so I don't know if my story is too complicated, but the conclusion I came to is that there is no such thing as a too complicated story. There is just bad writing. Arguably, that's worse news, but it also means that there is something you can do about it. If JK Rowling had dumped the whole plot of Harry Potter on us in the first chapter, it would have been highly confusing, but she didn't. She wrote it well, telling the story slowly. Getting us used to certain ideas before throwing fragmented souls and antisemitic villains in our lap. Your idea isn't too complicated, the way you're writing it might be.
Here's what happens when we try explain Harry Potter in more detail, including all the complexities:
Harry Potter is an eleven-year-old boy who lives with his nonmagical (muggle), mentally and low-key physically abusive aunt and uncle. Harry's parents were murdered by a Hitler-Esq wizard named Voldemort. As a remnant of the fear Voldemort engineered in the people of the hidden magical world, wizards and witches still refer to Voldemort as He Who Must Not Be Named.
During the attempted murder of his activist parents, Harry, a baby at this time, survives a killing spell because of his mother's love that is activated through the sacrifice of her own life to protect her son. Love being a powerful kind of magic that Voldemort does not understand and undervalues. The killing magic intended for Harry rebounds on Voldemort.
As a result of the mother's love that protects Harry until his 17th birthday, as such Voldemort cannot physically touch Harry. While this rebounded magic should have killed Voldemort, he had previously created Horcruxes, fragments of his soul that he hid in objects he believed held value. Making him essentially unkillable unless each Horcrux is destroyed. As a Horcrux is created through a complex magical process involving a murder, a fragment of Voldemort's soul is transferred to baby Harry Potter.
Being a soul storage facility, Harry can sense Voldemort's emotions and even see through his eyes on occasion. Harry is unaware that this is happening as he does not know that he a soul jar. Voldemort and Harry's wands also hold 'twin cores,' and as such, they do not work properly against each other. Harry attends the magical school Hogwarts. He has friends called Hermione and Ron. There are also dragons.
I know I use Harry Potter as an example a lot. That's because it's a good book, follows clear writing ideas, and is popular enough that most people get the references.
You Don’t Think Your Story Idea Is Original Enough
Who cares? Seriously. Who. Cares. Don't be original; originality is so unoriginal. If you're not copying or plagiarizing, then go forth and bring your unoriginal story to the world. Maybe your story has been told before, but it hasn't been told by you. I was recently reading an Andre Norton book, The Cycle of Oak, Yew, Ash, and Rowan (3 out of 5 stars, only recommend if you’re desperate-free on kindle unlimited). I had read it before when I was a teenager and wanted to revisit it—suddenly, I realized that it was a retelling of Snow White. I don't know if it was intentional or not, and I didn't make the connection the first time I read it. Did it spoil the book? Not at all.
So many books take inspiration from other sources, be it books or movies. Bridget Jones's Diary is Pride and Prejudice with an insecure protagonist packaging. The Lion King is Hamlet, but with Lions.When Harry Potter came out, my school's librarian fell into an impassioned rant about how nothing in the book was original, how it was all 'stolen' from other stories and repackaged. At that point, I hadn't read it—I automatically eschewed anything popular. I still do; it's ludicrous—that librarian rant was one of the driving factors in my decision to pick up the boy wizard book. Repackaging a bunch of other books ideas into one? Sounds cool.
If it worries you—and it shouldn't—then start writing and look for opportunities to subvert the story. While you're writing, your characters will likely develop a will of their own, let them rebel, and you'll find yourself telling a story all of your own.
You Have a Plot, but You Don’t Have Characters/ You Have Characters, but You Don’t Have a Plot
Some books are character driven; others are plot-driven. If your book is plot-driven, the characters will be a secondary consideration. It doesn't mean they aren't important. They aren't AS important. The internal world of your characters isn't going to be the primary focus, and you aren't going to need to delve as much into the psyche. I thought I wanted to write a character-driven book, possibly because my favorite books are the ones where almost nothing happens, but the characters are interesting. Turns out living in my character's mind while writing bores me to tears.
That's fine. I have to put more effort into it sharing their thoughts and feeling. It is a conscious thing I must remember to do to compensate for my natural style. Others will naturally spend more time with their characters and have to put more effort into detailing the plot and the world. Start writing with your characters or your plot and build the rest around those core elements. You might even find that your thinking and plotting style is character driven, but your writing style is more plot-centric.
You have ideas, believe me, you have them. You just need to go with the flow and lean into your skillset.
You Really Don't Have An Idea For A Book—Write Like A Pantser
Fine, I believe you.
A lot of books start from a scene, a character, or the fleeting idea of an idea. I started my book with a whole idea. I wrote the first 5000 words off that idea. Do you know where those words are? Dead and buried. Ideas are easy, but writing them is hard. That doesn't mean that you won't be able to write if you can't come up with an idea. It doesn't work like that.
If you are short on ideas, you might be a pantser rather than a plotter. I'm a pantser myself. I didn't have an outline, nothing put to paper when I started writing. Characters came into existence, not because an outline told me they needed to but because the chapter had gotten boring, and I wanted to interject someone new. If anything, coming up with an idea and then trying to write is a disaster for me, I have to come up with it on the fly, or nothing works.
Sit down and write, an idea might be forming, and you don't even know it.