Before you add the poilcy that makes it possible for the DigiExam app to start in autonomous single app mode, make sure that the DigiExam app itself is added to your MDM-system.
Add autonomous single app mode policy
1. Go to Policies
2. Select "Restrictions"
3. Scroll down until you find "Autonomous single app mode permitted app ids"
4. Click on the "+" symbol and write se.digiexam.ipad in the text input box
5. Scroll all the way down and click on "Save" in the bottom-right corner
6. Re-sync all iPads in the group(s) that you have added the policy and the DigiExam app to