As per the description above, you want to add a new license to an existing account, Is that correct?
The record shows that you are subscribed to an Individual subscription of Acrobat Pro DC on your current Adobe ID, and this can be used by one user only who placed the order. If you wish to add a new license, you can place a new order of Acrobat Pro DC on the user's email address.
Or you can also subscribe to Team subscription of Acrobat Pro DC Creative Cloud pricing and membership plans | Adobe Creative Cloud
Feel free to update this discussion in case you need further assistance.