This error usually means that the student has clicked on an assignment that was generated by copying a link to an assignment. This may be a link to an assignment another teacher has made, or one from another class of yours. We also sometimes see it if teachers have used the "Reuse Post" option in Google classroom.
Due to limitations of the Google Classroom API, copying links to assignments, or using the "Reuse Post" option won't work. The links in CK-12 assignments made to Google Classroom have class and assignments identifiers in them, so when students click on those they are going to the original assignment and class, not to an assignment in the class in which you pasted the copied link.
If you have made any of your assignments this way you will need to delete them, and make assignments following the steps in this Help Center article:
We will also sometimes see this if the student has tried to access the assignment with an account that isn't actually in the Google Classroom to which it is assigned.