Android+ Application Catalog mandatory apps not installing
Mandatory apps items from Amazon app store will not be silently installed on any Android+ devices. Even for Android-Enterprise devices, only mandatory apps items from Managed Google Play store (bit not from Amazon app store) will be silently pushed. I don't think you have any alternative other than manually initiating installation of the app.
I'm trying to download Amazon Relay from the Google Store.
The application is in the Application Catalog as a Google Play Store Application rather than a Managed Google Play Application.
When I tried to send the application as a Managed Google Play Application, I'm guessing this option is for Enterprise devices?
As said in earlier post, mandatory app item will not be silently pushed from personal Google Play store for Android+ device as in your case. Your device must be recalled, factory reset and enrolled an Android-Enterprise Device-Owner mode, and then your required app set up in your managed Google Play store can then be silently pushed to the device.