Error: A credential 'Administrator' is already associated with the domain

[3.23 and below]

This error happens when a new domain credential is added. It is not possible to create a duplicate domain credential. The domain credential will access all devices registered in the domain, so it is redundant to have 2 or more of the same credentials in the same domain.

In order for the credential to have access to a device, just add this device to the domain, in the editing screen of the same.