Most character names have already been taken, but if you're writing a fanfiction with an OC as the main character, or just a book in general, there's a good chance that the name you chose has already been taken. But my advice? At least try to make your character's name original. I mean, here's an example:

If you're writing a Harry Potter fanfiction, then don't name your main male character Harry, unless it's actually Harry Potter. It's not unlikely that there would be two Harrys at Hogwarts, but it gets confusing.

If you have a female lead, name her something like Eleanor, or Emma. Those two names are actually used in the books, but they're only used once, so you should be safe.

Also, if you're wondering where these two characters appear, both Eleanor Branstone and Emma Dobbs are being sorted in Harry's fourth year (The Goblet of Fire).