How to resolve Could not load file or assembly 'Xamarin.Forms.Core, Version=2.0.0.0

Since updating my PCLs to use .Net Standard 2, my Android build is reporting the following. Can anybody suggest how to resolve this please? I am using VS 2017 Community v15.9.13 . My iOS and UWP builds work fine.

Using "RegisterAssembly" task from assembly "Microsoft.Build.Tasks.Core, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a".
Task "RegisterAssembly" (TaskId:631)
  Task Parameter:Assemblies=G:\tfs\myapp\myapp\AndroidForForms\AndroidForForms.Android\bin\Debug\AndroidForForms.Android.dll (TaskId:631)
  Task Parameter:TypeLibFiles=G:\tfs\myapp\myapp\AndroidForForms\AndroidForForms.Android\bin\Debug\AndroidForForms.Android.tlb (TaskId:631)
  Task Parameter:AssemblyListFile=obj\AndroidForForms.Android.csproj.UnmanagedRegistration.cache (TaskId:631)
  Task Parameter:CreateCodeBase=True (TaskId:631)
  Registering assembly "G:\tfs\myapp\myapp\AndroidForForms\AndroidForForms.Android\bin\Debug\AndroidForForms.Android.dll" for COM Interop. (TaskId:631)
C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets(4786,5): error MSB3217: Cannot register assembly "G:\tfs\myapp\myapp\AndroidForForms\AndroidForForms.Android\bin\Debug\AndroidForForms.Android.dll". Could not load file or assembly 'Xamarin.Forms.Core, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. The system cannot find the file specified.
Done executing task "RegisterAssembly" -- FAILED. (TaskId:631)