Outlook 365 - name cannot be matched to a name in the address list

I am not sure this is the place to ask. If not please tell where is the appropriate place.

I have tried creating a new email address in Outlook (Office 365) and it goes through the motions of auto detecting it but always pops up an error that says "the action cannot be completed. The name cannot be matched to a name in the address list"

I have searched high and low for a solution, some folks say it is a windows credentials issue. I have tried removing various entries there with no avail. Some have pointed to working through something called Exchange Admin Center, but that is not something I can find.

I'd be grateful to get some instructions to fix this.