The Assignments tool data schema was refactored between Sakai 11 to Sakai 12.

NB: For this reason Assignments exported in the Site Archive from Sakai 11 or earlier will not import into Sakai 12.

Assignment conversion (11-12)

•  After running all of the database conversion scripts you will not have any assignments in the assignment tool.
•  You should login to your system as admin, go to the Job Scheduler tool.
•  Click the Jobs tab, then New Job
•  For the name use "Assignment Conversion" and select the type "Assignment 12 conversion". The defaults should be sufficient.
•  Click on the button Trigger(0) next to this job, and click Run Job Now.

This process should convert all previous assignments over to the new format. It may take from 1-4 hours depending on how many assignments you have. You should only need to run this once but it will not re-process any assignments. See https://github.com/sakaiproject/sakai/blob/master/assignment/impl/src/java/org/sakaiproject/assignment/impl/conversion/README.md for more information.