Students who access an assignment but can't find a button to upload, attach, or submit a document most likely saved the document as a draft. This is common with Group Assignments on Compass, where one group member began a submission, but saved it as a draft without clicking Submit.
The solution is to click the Continue button on the right side of the screen. The student will then be brought to a screen where they can edit their draft and complete their submission. See screenshot below.