The {user} tags are not being converted

The Users Anywhere plugin is not published

Check if the Users Anywhere plugin is published. Go to Extensions >> Plugin manager and search for the system plugin System - Regular Labs - Users Anywhere. Then publish it.

There is markup on the {user} tags

If you have copy/pasted code directly into your editor, you might also have copied along hidden HTML tags. So this will also cause the {user} tags to break:

{<span>user</span> ...}

Remove the extra styling or other html tags around and in the {user} tags that might cause problems.