If you started your current session with an iOS app for which iOS Entitlements Support is turned OFF, then you can only install additional apps that also have iOS Entitlements Support turned OFF. Apps with iOS Entitlements Support turned ON are disabled for that session.
To test such disabled apps, you can either
- Launch a fresh session with that app, and carry on testing as usual
- Turn OFF iOS Entitlements Support for that app and try again