Book 8 makes a subtle but severe distinction: suffering by itself is not indemnity. If a person walks the path while centered on grievance and self-assertion, the heart-condition of indemnity breaks down.
That is why the text says complaint disqualifies the worker. The issue is not that pain is forbidden. The issue is what center the pain is organized around.
In this logic, indemnity is not merely hardship endured. It is hardship offered while still facing toward God. Complaint turns the attention back toward the wounded self and changes the meaning of the course.
This gives gratitude a more serious role than mood management. Gratitude is one of the ways the heart remains aligned while carrying something bitter.