Genre doesn't mean what you may think it means. Genre is play, prose, poetry.
Sub-genre is a category within one of those genres.
Shakespeare wrote in the genre of plays, he wrote in the sub-genre of tragedy, comedy etc.
Byron wrote in the genre of poetry, he wrote in the sub-genre of romance, melancholy, horror etc
J.M. Barrie wrote in the prose genre and wrote in the sub-genre of fantasy.
That's not particularly useful to know, but it's quite interesting! Something that is important to know, however, is that Wattpad's sub-genres are not the only choices for you.
Your story can be about anything you want, anything. So your sub-genre can be anything- it can be a mix of two or three or five sub-genres. You might write a sci-fi horror romance with adult themes in the form of literary fiction(literary fiction is a book that focuses on the way in which it is written. George Orwell's 1984 or Animal Farm is literary fiction). It's whatever you want it to be.
If you're not sure what genre you write best, then try them all. What do you enjoy reading the most? Try writing that. You will find your niche, and it will become your forte. We expect horror from James Herbert, fantasy from J.K. Rowling, romance from the Austen sisters, and so a signature sub-genre will eventually be attributed to you if you so choose it.