Hi Nadine, might the fix below help?
https://moodle.org/mod/forum/discuss.php?d=416189
Dis-allowing and re-allowing the "Submit assignment mod/assign:submit" capability in the Student role. Turn it off an on again then try the student view again to see if the Add Submission returns?
https://moodle.org/mod/forum/discuss.php?d=416189
Dis-allowing and re-allowing the "Submit assignment mod/assign:submit" capability in the Student role. Turn it off an on again then try the student view again to see if the Add Submission returns?