If the 'Require that students accept the submission statement' setting for an assignment is set to Yes but the Submission statement in Assignment settings is empty, students are unable to submit the assignment. On the Confirm submission page, clicking the Continue button only refreshes the page and does not submit the assignment. 

This is already solved on LMS by MDL-78564 for 4.1+ sites

On LMS
Steps to reproduce:

1. Log in as admin
2. Add an assignment to a course. Set the following submission settings:
   1. Require students to click the submit button - YES
   2. Require that students accept the submission statement - YES
3. In the admin/settings.php?section=modsettingassign page set assign | submissionstatement as empty and save

Or you can use master > [MoodleMobile Testing]> Assignment > Assignment requering statement - no drafts

On the app

1. Log in as the test user
2. Navigate to the assignment created in step 2, tap Add submission,  upload a test file, then tap Save
3. You will be ask to confirm, do so, you will see a loading and arrive to the assignment where you can edit the submission or submit it, tap on 'submit assignment' button and you will see an error message 'Please accept the submission statement'.