You trying to install iOS apps on the managed devices and you get this error after app distribution.
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 occurs when the user declines app installation by clicking cancel when the app installation is initiated. You can prevent this error, by installing apps without requiring App Store account as explained here.
This occurs if the user repeatedly enters wrong iTunes credentials or if the device cannot access *.apple.com, which is required for initiating app installation. You can prevent this error, by installing apps without requiring App Store account as explained here.
The device requires access to *.apple.com to initiate installation and further the region selected in the App Store might not have the distributed app. So, ensure the device can access *.apple.com and the correct App Store region is selected on the device. You can verify/modify the App Store region as explained below:
This also occurs if you are distributing apps through VPP and the app licenses are inadequate. Additionally, this error also occurs if there is a device type incompatibilty(distributing iPad mini apps to iPad) or OS incompatibility(distributing iOS 8 only apps to iOS 9 devices). The iTunes page of an app has all the compatible devices and OS listed as seen here.
If the app is already installed on the device, remove the app manually and install the app from App Catalog.
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.