Again issues with translation of 作业, homework was not accepted but the solution was assignment.
I reported that exact same thing weeks ago! I guess there are quite a backlog of reports they have to work through.
What was yesterday's homework...
Model answer gives "homework FOR yesterday". That means the homework that had to be turned in yesterday. Homework FROM yesterday makes more sense here
"What were yesterday's assignments?" should be acceptable.
作业 is translated by Duolingo as 'homework ' or ' assignment' but they are for some unknown reason not considered interchangeable. Either of these should be acceptable.
What is yesterday's homework should also work isn't it ?
What was yesterday's homework? - is accepted