Admin Tools Pro has a setting that prevents super user accounts from being modified or created - which is a very useful feature considering what hackers usually target
Thus you just need to temporarily disable that to make super user changes
Administrator -> Components -> Admin Tools -> Web Application Firewall - > Configure WAF -> Joomla! Feature Hardening Options -> Disable editing backend users' properties
(becomes 'No' to modify super user accounts - make sure you set it back again to 'Yes' after you've finished editing)
Save button - as almost always in Joomla - top right of screen
(becomes 'No' to modify super user accounts - make sure you set it back again to 'Yes' after you've finished editing)
Save button - as almost always in Joomla - top right of screen
