An incorrect implementation of UDP in your game client could prevent users from purchasing IAP products. Typical symptoms of such a problem in your game include:
The wallet doesn’t appear when invoked
In-app purchases are unresponsive If these symptoms occur in your generic UDP build, resolve them before you repack your game.
Make sure you push your IAP Catalog information in the Editor.
This can happen with an implementation via Unity IAP, if you forget to set UDP as the build target.
It can also happen if you use the Minify option while building your APK; UDP may not be able to find files/directories that it needs because of it. Keep UDP-related packages in a customized proguard file (or disable Minify option) and rebuild your game.
The login screen in the Sandbox can fail to appear if the Init() method has not been implemented properly. The login screen is displayed when Init() is called. Init() is successful only when the login is successful. Unity recommends to implement Init() when your game launches.