You can use classic editor to remove categories. You can also use Source editor in Visual Editor to remove categories.
one of my pages is in two categories it should not be in. I went into the classic editor, but it does not show it in the category section!
one of your page probably uses one of your templates that sets one of your categories. the solution is: remove the category from the template or wrap the category by noinclude tags.
Thanks so much!!! It worked!!!!!