When your Xamarin.iOS builds run from solution file (.sln), there are several things you might want to check. 

You should ensure Android and UWP projects are disabled in your code for build configurations that are intended for iOS builds. Go into the solution's configuration mappings, and for all mappings that target iPhone and iPhoneSimulator, uncheck all the projects that target different platforms. This will ensure when the .sln begins building, it will not attempt to build other projects. 

There is more solution configurations mapping information you can read.