What do I do if I get a message saying my assignment isn't prepared yet?

If a student receives the "Your document is being created. Check back soon" that means the PDF annotation tool did not load in time when the student accessed the assignment for the first time. 

How resolve this issue

Simply have the student re-open the assignment in Canvas or just hit the browser refresh button

Once the assignment is reloaded the student will see the PDF with the annotation tool as shown below.