Teacher: I'm not able to view or markup my student's assignments in DocHub

If your student has turned in an assignment in Google Classroom, but you are either unable to view or edit the document in DocHub, your students should set you as a Contributor before (or after) turning it into Google Classroom. This way, you'll be able to access it and grade it using markup in DocHub. 

Alternatively, you could just make a copy of it which will keep the student's work and let you add your own grading annotations in a new copy. To do this, just click on File menu > Actions > Make a Copy. Here is the screenshot:

Here is how your students can add you as a Contributor:

1. Open the document that needs to be shared

2. Click on the Share or Send ​​ button

3. Enter your email address into the bottom field as a Contributor and click Add

4. Close the Share window

5. Save it to Google Classroom and turn it in