Apologies for the inconvenience caused. In Prime once you delete any user then unfortunately you will not be able to add the same user back again . This is as designed.
The reason for this is, when a learner is deleted and becomes active after some time, it will mess up the reporting data, gamification strategy, certifications etc. for the time when he was missing from the user list. To avoid such scenarios, Prime does not support re-adding of a user.
In case you deleted the user accidently then we can try to reactivate the user back from the back-end. Please let me know if that is the case so that we can assist further. Also please let me know if you are using the Trail for Prime or you have purchased it?