Can’t create categories

I can no longer create new categories (either directly or through the post).
The timer just keeps spinning and nothing happens. No error messages, no red flags, etc.
I disabled all plugins. No change.
What else causes this?
None of the other answers that I saw in the forum work for me (although one answer mentioned a firewall, but with no instructions on how to determine if that is the problem or how to fix it).
Another suggested editing press_this.css, but no information on what that has to do with categories.
I have the latest version of WordPress and am using the new block editor.
Help!