Unable to add a user in free trial

I am getting the following error when I try and add two users to my Adobe Sign account: "You cannot add that email address to your account. Please see this Knowledge Base Article for more information."

Having read the article, I have verified that there are no other users in the system, other than the admin.  I am trying to add my manager's email to the trial as we test.

The other option says:

Some accounts are set up with domain level security. All site license customers should have this type of security, as well as customers that use SAML.

However, domain level security isn’t exposed in the Admin interface.  It can only be evaluated on the back end by Customer Support or the Customer Success team.

If the problem user isn’t in your account under any status level, then the problem is almost certainly a domain permission issue.  Contact Customer Support or your Customer Success Manager if you have one.

Be sure to include your Account Admin email and the email of the user you are trying to add.

This other option says to contact Customer Support, however, I am not entitled, due to being on the trial.

It will be a difficult sell to put money into a product, if I cannot get the trial functioning well. 

Please assist.