Each preparer entered in your program must have a separate individual PTIN. The default preparer's entry should be the practice owner. The practice owner must invite other preparers. If the preparer you are trying to invite is set as the Practice Owner, then they can't be invited. You need to be set as the Practice Owner, deactivate any extra preparers you may have created in the process, and then invite the preparer you want to invite.
The Practice Owner Preparer cannot be deactivated, and that title cannot be switched to another Preparer, so you have to edit the Practice Owner to make yourself the Practice Owner, then deactivate any other Preparers that you created with your name.
To determine which user is the Practice Owner in your TaxAct program:
To make yourself the Practice Owner:
To deactivate a preparer: