Admins will not be able to add the 'admin' role to other user profiles.
Release or Environment
All supported releases.
This mainly happens if there is an invalid entry in the Contains Role (sys_user_role_contains) related under the admin role.
It could be that a role was present under the contained role, which has now been deleted.
The relation is still there, and it shows as an invalid sys_id.
Remove the invalid sys_id and add the role again to the user profiles.