Actually…I can now add categories from a post (but still not from the Categories screen).
And a second issue has reared it’s head:
When I add a post I am taken to an error page:
Safari can’t open the page “https://tentoes.com.au/dev/wp-admin/post.php” because the server unexpectedly dropped the connection. This sometimes occurs when the server is busy. Wait for a few minutes, and then try again.
The post is added, though.