Hello @psypniew ,
This might happen due to when a plugin is activated that associates one or more application-specific admin roles with the system admin role. In this scenario, assigning the admin role requires another user who already has all the required scoped admin roles, not just the system admin role.
Try this :
Remove any application admin roles from the system-level admin role and assign them only to the respective application admin users. Once the system admin role is clean, admins will be able to assign the admin role successfully.
If this helps you then mark it as helpful and accept as solution.
Regards,
Aditya,
Technical Consultant
