Make sure that your phone is updated. Go to the settings on your phone to see the version of the operation system.
Read more about the technical requirements to your phone on MitID.dk/technical-requirements
Your passport/ID card must be valid.
Your passport/ID card must have this symbol. It tells that your pasport/ID card has a hidden chip and can be read by the MitID app - and confirm you identity.
In addition, check on this list whether your passport/ID card meets the requirements.