While adding a domain from DS-NOC, the following error is reported:
Unable to connect to Exchange: Cannot connect to Exchange with given credentials. Response status is HTTP/1.1 500 Internal Server Error

DIR sync was enabled in the environment causing this authentication failure.

An updated tool from Microsoft Azure AD Connect (that replaces the older tool DIR Sync) is required to enable single sign on.
Once single on is enabled, the domain was added successfully through DS-NOC.

The following links provide additional informaiton on Azure AD connect and its feature.