Apple doesn't provide explicit error messages when installing apps on devices. It can be quite difficult to understand what could have gone wrong.
By default on Android devices, installation is not allowed from "unknown source", in other words from sources/sites other than the Play Store.
In order to install an app from Diawi, this must be allowed through the following setting on the device: in the Settings app, depending on the device and Android version, it could be in Security or in Applications, check the box "Unknown sources".