I encountered this error when trying to build a Xamarin.Forms Android project:
##[section]Starting: Build Xamarin.Android project
MSBUILD : error : Fody: Could not find a weaver named ‘RealmWeaver’.
MSBUILD : error : If you have nuget package restore turned on you probably need to do a build to download the weavers.
MSBUILD : error : Alternatively you may have added a weaver to your ‘FodyWeavers.xml’ and forgot to add the appropriate NuGet package.
MSBUILD : error : This url may provide more information http://nuget.org/packages/RealmWeaver.Fody/ .