Why can't I schedule a post on my Facebook Page?

Usually it's either the place you're trying to post from or the access on that Page. Try creating it in Meta Business Suite rather than the regular Page composer, and make sure you're working on the right Page. Check Page access too: people with Facebook access can manage Page content on Facebook, while task access is for management tools such as Meta Business Suite. If the Schedule option still won't work, try a plain text test post with no cross-posting or extra media. That helps separate an access/setup problem from a problem with the specific post.