"The name you entered can't be used. The name is not available. Enter a different name." I'm installing El Capitan on a MacBook Air that I aquired with no internal drive, so no previous OS.

I proceed through several steps fine until "Create a Computer Account".



But it doesn't like the details I enter and shows this cryptic error box:


  The name you entered can't be used.
  The name is not available. Enter a different name.




The worst part is that it now does not allow me to edit any of the text fields! I can click on them and drag and select them, but I cannot type new letters or delete existing letters, etc.

Is this a known problem? I can't find a solution to it via Google though I do find a tiny number of people asking about it. If not, what am I missing?
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.