"User could not be created..."

[MaryB]

I am having trouble setting up one of my resources in PWA. I am setting him
up with Windows Authentication, in exactly the same way I have successfully
set up everyone else. For some reason, every time I try to set up his
authentication and permissions, it tells me:
The user could not be created. Check the spelling of the user name, veryfy
that a valid domain name was included, and check that a duplicate domain was
not used.

I have done this. I don't understand why I am unable to set up this one
particular team member.

Have any of you had this problem?

Thanks!