What Do I Do If Test Cases Cannot Be Associated with Work Items of the Task Type?
Symptom
In a Scrum project, work items of the Task type cannot be associated when associating a requirement to the test case.

Root Cause
In a Scrum project, the Task type is more oriented towards a specific development task than a complete requirement story.
Therefore, only the Epic, Feature, and Story work items of a Scrum project can be associated with the test cases. Work items of the Task and other custom types cannot be associated with the test cases.