Does a story in fan fiction have to be related to a movie or book to be posted or can it be a plain story that I came up with😳

Ok so if this is a stupid question you can smack me just post *smack* with your response and I'm pretty sure I'll feel it...like...literally. Anyway I have an idea for a story(actually a lot of stories) and at the moment I dont think it's related or connected to any book or movie so idk if I will be allowed to post it. Can I?? I believe in the future i will post stories related to other previous works but right now I need to get the these stories out of my head so my mind is clear and can come up with a actual fanfic but idk if I would be able to post the stories.

Note: Idk any better because I made my account when I believed the ideas would work and now I'm not so sure😥