"The name you entered can't be used. The name is not available. Enter a different name."
I ran into this issue today. Apparently the error message is trying to tell you that there is an existing person with the same "Full Name" as you. Hence it cannot let you add another username with the same "Full Name". In my case, I was recovering from Time Machine and getting this error msg. So I just changed the full name to my last name. Was able to create an account using that and proceed further.