There’s a terrible situation in Unity when users get stuck while trying to switch to Android platform. Let’s look how to fix that.
If you can’t set Android platform it means you did not install the Android Build Support package. One of the main reasons for that is the Unity update process. When the Unity Download Assistant runs it does not include the AndroidBuildSupport in the default packages list to install.
Setting Android platform in Unity editor
So now you should run the Unity Download Assistant again and select the Android Build Support and install it.
If you don’t have a Download Assistant you can get it from the official site.
When Android Build Support is installed you can switch to Android platform in the Build Settings menu.
If you did not set up the Android SDK you should do it in order to run your app on Android devices.