When using "Switch Role to..." in order to see a student view, the [Add Submission] button will not show up if you are not ENROLLED in that course (See image 1). This issue usually only arises with Managers and Administrators who are able to view courses without being enrolled as a teacher or a student.
There's no need to worry about it, though. The button will be there for the students of the course (See image 2).
Image 1 (the view of anyone NOT enrolled in the course):
Image 2 (the view of anyone enrolled in the course):
