The error is:
Error XA0134: The application does not have the 'android:debuggable' attribute set in the AndroidManifest.xml.
This is required in order for Fast Deployment to work. This is normally enabled by default by the Xamarin.Android build system for Debug builds. Please check that you to not have this attribute set on the 'application' element in your 'AndroidManifest.xml'.
If you have a class that derives from 'Android.App.Application' and are using the '[Application]' make sure the 'Debuggable' property is not set at all as it will override the value for debug builds.
It is not clear how this can be overcome.
I have tried including "android:debuggable" within the application element within the Manifest (which the error says NOT to do, I know). The error persisted.
I have tried including the following in the AssemblyInfo.cs:
#if DEBUG
[Application(Debuggable = true)]
#else
[Application(Debuggable = false)]
#endif
The error persisted.
As I mentioned, it is not clear how to fix this and answers to similar questions are years old.