First, at the risk of being picky "the app" doesn't ask for a MRN. It needs to be part of her Church Account.
Make sure she has the correct MRN. What's written on her temple recommend could be incorrect.
Lastly, the most common problem is that the member already has an account they've forgotten about. Perhaps created for FamilySearch, Seminary, BYU, etc. They'll need to recover that account.
Have you searched the Help Center? Try doing a Google search and adding "site:churchofjesuschrist.org/help" to the search criteria.
So we can better help you, please edit your Profile to include your general location.