gg987
You wouldn't be able to right-click the Category itself in the sidebar to delete it – you would need to remove all of a Category's items first for it to disappear on its own.
However, as it turns out, it looks like empty Categories are sticking around when they shouldn't be, which might explain what's happening in this case. Sorry for the inconvenience. I've added this thread to our internal discussion about it. Our development team are already aware of this happening, and will hopefully have a fix for it in the near future. Keep an eye on the 1Password Releases blog to see what's new and what's fixed.
ref: dev/core/core#13984