The reason you can't add Service Reference in a Xamarin Forms PCL project can be:
Windows Phone 8.1 (not Silverlight). Remove it by going to properties on PCL project and click Change. You might not be able to do it before you remove all your Nuget Packages.
Visual Studio crashes when you add the service reference. Remove Xamarin Forms nuget package and Asp.net SignalR. That worked for me. I didn't have Entity Framework Core but that might have to be removed temporarily as well.