Occasionally, a student or two may not get a copy shared to them when you're assigning a copy of a Google Doc to every student.
In order to verify that permission, have the student click "Request Access" from within their assessment.
Once the student has requested access, the main teacher will log into their gmail to find an email from Google letting them know that the student is requesting permission to edit a document titled "'Your Document'-'Student's Name'".
Open that email, and click on "Open Sharing Settings".
Have the student refresh their web page, begin the assessment again, and the document will now be shared and embedded within the Otus rubric-based assessment.