Your project is not referencing the "MonoAndroid,Version=v8.1" framework. Add a reference to "MonoAndroid,Version=v8.1" in the "frameworks" section of your project.json, and then re-run NuGet restore.

I had  the same problem.

Reason: file obj/project.assets.json target "MonoAndroid,Version=v7.1" when my project target android version 8.1
I resolved it by:

1. Right click Android project => Properties => Application => Compile
   using Android version => Android 7.1
2. Clean and Build project => build success, and restart IDE.
3. Right click Android project => Properties => Application => Compile
   using Android version => Android 8.1
4. Clean and Build project => build success

File obj/project.assets.json updated to "MonoAndroid,Version=v8.1"