Sometimes there is a mismatch in dates between Canvas and Turnitin Feedback Studio. The error message below is the identifying symptom of the problem.

Turnitin Error

Sorry, we could not process your request.

Turnitin assignment could not be created.

date_post — Post date must be after start date 

To fix the conflict: 

1. Edit the assignment settings in Canvas, temporarily removing the After (Start on the Tii side) date. Save the change.
2. Open the Turnitin Assignment from Canvas. 
3. Select Settings. 
4. Change the Feedback Release Date so that it is at least one minute after the desired After/Start date. 
5. Save the changes. 
6. Edit the assignment settings in Canvas, restoring the desired After/Start date.
7. Save the changes.