To add users to a Task you can either add the names manually in a colon separated format wrapped in quotes:
Or click the magnifying glass and search for them specifically:
These users still have to exist in EEM if you enter them manually, the search option will create the list for you, but it does not automatically show users until they have been searched on.
Back to the error - if you go into EEM and assign the user the 'pamuser' permission policy specifically, does that allow the task to be completed?
If not then who was the task assigned to? If a task is assigned to a specific user, only that user or a PamAdmin user will be able to complete that task unless the original user delegates the task to a coworker.
For example, If a process assigns a task to 'Bob' but somehow 'Jill' gets the link to that task and tries to complete it Jill would see a similar message about not having permissions unless Bob delegates:
but, if Jill was a member of the PamAdmin policy in EEM she would have permissions to take and complete, or delegate the task for Bob.