Fix error: You must specify user account name when adding a DC to an existing Forest

When the following error is prompted “You must specify user account name when adding a DC to an existing Forest”

Adding DC to Existing Forest: User Account Name Error Fix

The problem here was that Windows refused to use the default administrative credential.

Solution: 
- created a new user account in the domain
- And added the user as a Domain administrator
this resolved the issue and I could get passed this state