I am doing the "Create a Guest Administrator and deactivate it" challenge in the "Data Security" module.  The challenge is to "Create a new user using the System Administrator profile and then deactivate that user".  However, I cannot create any System Admin users.  The "System Administrator" profile choice is not available when I create the user, probably because I am using a free developer account.  I can't continue with the challenge.  Does anybody have this problem?

I am practicing on trailhead via using my developer org. The chalenging asked me to Create a Guest Administrator and deactivate it. I am currenly the System Administrator of my developer org. We have only one license for System Administrator. How should we solve this issue in order to:
Create a new user using the System Administrator profile and then deactivate that user to preserve the licenses in your org.The new user must use the System Administrator Profile.
The username for the new user must include 'guestadmin' somewhere in it.
The new user must be inactive.
I need some hints or advices. Thanks.