Steps to replicate:
Pre-reqs: course with student(s) enrolled.
- Create a moodle assignment - default settings should be fine
- Have a student view the assignment and confirm you can see the 'Add submission' button
- As the Teacher go into the grader report
- Turn editing on in the grade report
- Lock the grade item for the assignment created in step 1
- Log back in as the student and view the assignment - there will now be no 'Add submission' button
I'm unsure of what the actual expected outcome here is as I can see it a couple of ways.
a) The grade being locked shouldn't stop the submission of an assignment - but the grading of the submission.
b) If the submission should be stopped by the locking then their should be some sort of notice on the submission page that the submission are locked (similar to if they are locked through the assignment module).