You trying to install iOS apps on the managed devices and you get this error after app distribution.
Provide access to *.apple.com as it is required to initiate app installation. This error also occurs if the user repeatedly enters wrong App Store. You can prevent this error, by installing apps without requiring App Store account as explained here.
If the size of the app that you are distributing to the devices is over 200 MB, then Apple does not allow these apps do be installed over mobile data. The device must be connected to a Wi-fi to initiate the app installation. Use either of the following steps check the app size:
If the app size is over 200 MB, ensure the device is connected to a wi-fi to initiate the app installation.
If the apps have already been installed by the user, and again distributed by MDM, this error occurs. In case of supervised devices, the apps are automatically managed when the device is scanned. In case of unsupervised devices, if the app is available in the App Catalog of the device, clicking the option to Manage via MDM will bring the device under management. You can also choose to distribute the app with the option Silent Distribution enabled. If the apps are still not managed by MDM, try deleting and re-distributing the apps to the devices.
The various iWorks apps like iMovie, Pages, Numbers, Garageband and Keynote are preinstalled in devices with storage size greater than 32 GB. These apps cannot directly be managed by a MDM solution and hence will have to first be removed from the device and then distributed from MDM.
If the app has been removed from the App Store by the developer, you cannot subsequently distribute the app to devices/groups. Click here to know how to identify apps not present in the App Store. Also, the app you are trying to distribute to devices might be a 32-bit app which is not supported in devices running the latest OS version. Click here to know how to identify 32-bit apps.
When an app is distributed to unsupervised devices, the user will be prompted to initiate the app installation. If the user declines the installation by clicking cancel, this error occurs. You can prevent this error, by supervising the device and installing apps without requiring App Store account as explained here.
Ensure the user has an App Store account added to the device. Users may not be able to add their App Store account if the restriction Modify Account Settings (present under Restrictions -> Advanced Security) has been enabled. Disable this restriction, to allow users to add their App Store account. You can install apps on the devices running iOS 9.0 or later versions, irrespective of whether the users have signed in with their App Store account or not if the apps are purchased via VPP.
This error could occur on the device due to any of the following reasons
The iTunes page of an app has all the compatible devices and OS listed as seen here.
Sometimes the App Store region that distributes the app is not selected on the device. Follow the steps to select the correct App Store region on the device.