Your students will see this message if they try to open an assignment in WebAssign that they have not previously opened by clicking the assignment link in Brightspace®.
Resolution
Your students need to open each assignment first from Brightspace.
Opening the assignment first from Brightspace allows the assignment score to sync to the correct Brightspace activity.
If students see this message after opening an assignment from Brightspace, see Students Can't Open WebAssign Assignments From Brightspace.