I had done by task in control access to the org ,i have created guestadmin user but i got an error ,to edit that error i have opened again ,in that i just clicked on one box which appears on beside the edit button,then onwards its not appearing in org .

Hello@Asha Kinthada,

• Once user is created you cant change username as per error message.
• create new user  :-As per requirement  
  • Username: must contain guestadmin somewhere in it
  • try to create new user  with user name like  

ashakinthadaguestadmin1234@gmail.com 

Please check prerequisite :-Before You Start

• Important: Use the new Trailhead Playground that you created for the hands-on steps and challenges in this module. Using an existing Trailhead Playground might cause problems completing the challenge.
• Visit Setup | Company Settings | Company Information | User Licenses to verify that you have one remaining Salesforce license available.
• If you already used both of your Salesforce licenses, deactivate one to complete this challenge.
• https://help.salesforce.com/s/articleView?id=sf.users_permissionset_licenses_view.htm&type=5