You may see the following error from within the SimpleMDM iOS app:
A configuration file was not found. Please install this app via the SimpleMDM service.
This error most often occurs when the SimpleMDM iOS app is installed manually from the Apple App Store. The SimpleMDM app must be pushed to devices by the SimpleMDM administrator, otherwise, the app will not receive a special configuration file that informs the device of how to identify itself with your account.
To resolve the issue, delete the SimpleMDM app from the device and push the SimpleMDM app to the device from within the SimpleMDM administrator interface.