Please make sure that you have given the Administrator the System.Administrator role in the App security, and it has the right to create users with different user roles.
Let me know if you face any issue,
Hope it helps!
Is this option checked?
I gave System.Administrator role and right to create users with different user.
Actually in Demo/Prototype security I can access password attribute and create new user but after switching its password attribute of System.user is not accessible