Username: must contain guestadmin somewhere in it

I attempted asking this question in a topic already created for it but it did not receive any responses therefore, I am reposting for relevancy.  

When creating a new user for the 'Control Access to the Org' challenge (within the Data Security module), I am unable to deselect the 'Active' radial box to create an inactive user. I am also unable to create a 'guestadmin' user. You'll find the URL for the challenge I'm referencing below::

https://trailhead.salesforce.com/content/learn/modules/data_security/data_security_org