Have you impersonated the system user (or given it a password and logged in to it, if that’s possible) before trying to add admin users? If so, you could try logging out and back in as your normal (admin) user.
Have you impersonated the system user (or given it a password and logged in to it, if that’s possible) before trying to add admin users? If so, you could try logging out and back in as your normal (admin) user.