Unable to install iOS app

In case of devices running iOS 8.0 or lesser versions, App Store must be present on the device to install apps(both manual installation or through MDM).  For devices running iOS 9.0 or later versions, you can install apps without requiring App Store if the apps are purchased via VPP.  Verify if the App Store has been restricted by MDM. If yes, enable it by navigating to Restrictions -> Application -> Users can install apps through App Store. Ensure the user has not disabled App Store through restrictions present on the device. You can view these restrictions by navigating to Settings -> General -> Restrictions. 

User has not logged in with the App Store account

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.

User declined app installation

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. 

Device cannot connect to App Store

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.  

App cannot be downloaded on the device

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:

• On the manged devices, go to Settings and click on iTunes & App Store
• Select the existing Apple ID and click on View Apple ID. Enter the password when prompted.
• Now change the Country/Region, to ensure the distributed app can be installed. 
• Click on Done to save the changes.

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. 

App already installed on the device, but not managed by MDM

If the app is already installed on the device, remove the app manually and install the app from App Catalog. 

App not present/incompatible

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.