Issue
This issue happens when you are trying to use fast deployment on a device which does not support it. In this case both the normal and backup types of fast deployment failed. The Fast Deployment system makes use of the run-as feature of the Android OS. This feature was either not available or had limited capabilities in API 20 and earlier
解决方案
Disable Fast Deployment by setting EmbedAssembliesIntoApk = True in your .csproj.
或在 IDE 中关闭 Fast Deployment 。 你仍然可以在设备上调试，所有必需的文件都将打包到.apk中。