When trying to add an email for Access/user into an account, we are receiving the following error and not able to add:

User <Trying to add> is already associated with old account and can not be added. Error processing Data email already used.  

It is due to the email already associate with another account.

CA App Synthetic Monitor

Raise a support ticket referring to this knowledge article and ask for one of the engineers to review what is happening with the mail that you want to add.