Ideas are cheap, but that's not bad
This post should take about 3 minutes to read through.
One of the biggest mistakes writers who are just starting out make is to believe that since they had an amazing idea, they'll write a successful book.
The imagination is a muscle. If you regularly expose yourself to amazing ideas and learn to notice them, you'll see incredible ideas everywhere, which is why reading is so important. Additionally, the more you try to come up with ideas through brainstorming, the easier they are to come up with.
I first noticed this while running Dungeons and Dragons games as a teen. Me and my friends all started playing together at the same time, but they often needed two whole sessions to come up with a character that was interesting to them. In the same time, I could come up with twenty or more. In fact, my head was too full of characters that I wanted to play or to have my players encounter.
The same holds true now. I can come up with ideas for stories off the top of my head, and over the course of a week, I can develop as imaginative a world as I want. I have several hundred ideas stored in several notes apps I have used throughout my life—which is another way to exercise your imagination, writing down your ideas and examining them. (Please note, I do not consider myself special in any way. Any writer that's slightly above amateur will be able to do the same to some degree.) However, it's not these ideas that are important.
Consider the following: you're watching a science fiction movie, and one character says something in a slang that doesn't exist in real life but perfectly fits the movie's world. This is interesting and, if you're paying attention and catch it, might make you go "huh, nice." But will it make you tell a friend you saw "an amazing movie?" No. What will make you say that?
What is important in a story? It's how engaging the story is, how much it grabs your attention, how addictive it is, how impossible you find putting down the book. Ideas are important, otherwise the story would feel flat, but they're not the most important thing in a story.
I don't want you to feel disheartened by reading the above, but the exact opposite. Writers who are just starting out tend to put a lot of importance on ideas, and stress over how good or bad their ideas might be. They believe that they "just need the right idea" to make it big, or that if they fail, it's the idea's fault. They often believe that ideas come to them from some mystical place or force, and as such, they put their book's success in that same mystical force's hands.
The truth is the opposite of that. You can take a bad idea and make it into a great novel, as long as you put in the ingredient that will make your book succeed: work. It's sweat that makes a book great, hard work, writing and reading and studying about writing, and even more importantly, studying yourself. What do you find engaging? What makes your heart beat faster when reading? What makes you unable to put down a book, or unable to stop talking about it to your friends? Write that.
Additionally, ideas don't come from some mystical force. One's imagination is the source, and you can exercise that by using it in coming up with ideas, and feeding it with great ideas. I'm a visual writer, and one of the best sources of inspiration I've found is artists' twitters, and artstation. Make sure to exclude AI slop. I'll write more on how to exercise and improve your imagination at another date.
Ideas are cheap, and they will not make or break a story, so your great idea for a book might not guarantee its success. But you can guarantee your success by writing as much as you can, as well as you can, and creating a book that is as engaging as you can make it. Your first book might fail, but your fifth one might bring in enough to quit your day job.
Keep writing.