I had the same issue and despite installing Oreo (API 26) and ensuring the project target was the same it still didn't work.
In the end I updated the project file entry as it wasn't reflecting the… (View Post)
Didn't solve it but the following may be useful:
* occurs after installing latest stable updates for Xamarin studio 6.3 (build 863)
* problem occurs when running new or existing app on Android … (View Post)