Admin Tools Professional contains a feature called Web Application Firewall, or WAF for short. One of the security features that can be applied by WAF is to block editing or adding Managers, Administrators and Super Administrators.
Please go to and click on the button. In the new page, click on . Find the Disable editing backend users' properties option and set it to No. Now you can create and edit all users on your site. For security reasons, remember to re-enable this option after you're done adding/editing users!