Kelly,

The message, "The user you selected could not be impersonated, typically this is because they had a blank login id" is actually referring to a missing User ID (user_name) field on the user record.   It's this User ID that you type in along with your password to log into ServiceNow.   Without this field populated, a user is essentially just a contact without rights to log into the system.   Therefore, since they can't log in, you cannot log in as them via impersonation either.  

You will also be prevented from impersonating users who have Locked Out = true or Active = false.

I haven't seen the message you're referring to in any other cases.

Hope this helps.

Stacey