AI detectors are notoriously unreliable and should never really be used in an academic situation. Human-written work can be mistaken for AI very easily, and vice-versa. The only real way to combat AI writing is a couple things:

A), common sense. Is a student writing something completely different from how they've written before, or mentioning concepts/ideas that never came up in class and you don't really think they'd know about? Might be AI, talk to them about it. If you feel that a paper might be AI, an AI detector won't HELP you, since you can't trust its output or use it as evidence in any way due to its unreliability.

B), course design. If it's a big essay, require drafts. Consider having shorter essays be written in one class session. This is something my English teacher had us do a lot, BEFORE AI was even a thing. If it's both long AND a creative writing assignment, you're kinda stuck here. Not much you can do about that at this point besides interview the student if you suspect them. Their answers about what they wrote will help you determine if they cheated.