error when upgrading 5.4.5 to 5.5..1

This error is not major : it occurs when creating an index to avoid dupplicate work entry for same day (you should have only 1 entry per day fore a given assignment).
In your case, it seems you have a dupplicate on 2016-05-12 (may the 12th).
What you can do manually is to go to given week, remove all work for this day, save, close and reopen screen, remove again work for this day if some more appears, and once again until no more work exists for thos day, and the reenter correct work.
Then you can reexecute update (in phpMyAdmin for instance) :
We try and include a fixing patch in V5.5.2.