Checked this problem with our customer succes manager from Mendix. The documentation was outdated and did not match mendix 8. The academy recently received an update. This issue is now fixed!
Try checking the Access Rules at the Entity level (Course). Confirm that the appropriate user roles have Read/Write access. The screenshot you have above looks like its just showing those user roles have access to the page. Need to make sure they have permission to edit the field at the database level too.
Go to the Entity properties and find the “Access rules” tab. Then ensure the User Role you want to be able to Write to the field has “Read, Write” selected for the Duration attribute
If you get really stuck you can always download the module from the resources tab of the learning path. Nice to check your solution against it as well.
Hope that helps!