WordPress says category slug is already in use
When you try to create a new category, WordPress appends "-2" to it saying "The slug category is already in use by another term". You have gone through all of your categories to make sure you already did not create the category before and find that this is the first time you creating it.
It is not a WordPress bug, it's just way it works. All categories, pages, tags, links, etc are WordPress terms and occupy the same namespace, meaning their slugs can conflict with each other.
Hopefully, your issue is caused by a conflicting a tag slug; to fix that go to Posts > Post Tags and delete the conflicting tag. If it's a conflicting page slug, rename or delete the page. If it's not caused by a conflicting tag or page slug, you will need to manually delete the conflicting term from the "terms" WordPress table usually named "wp_terms".