As per the message, the user already had an Acrobat Sign account with the email address you used to add this user to the company's account. The backend team can help you make the necessary changes in this case.
We see that you have the Acrobat Sign Enterprise plan. Please contact our Adobe Sign Enterprise support team to get the changes done at the backend. You may contact them using the steps in the following help document: https://adobe.ly/3TxL6SK.