I’ve literally just had this with a brand new Xamarin.Forms application in Visual Studio 2017.
The root cause appears to be that I let VS2017 create the project in it’s default location ‘C:\Users\Dave\Documents\Visual Studio 2017\Projects’ and this has resulted in one of more files now having a path which it too long.
I moved the solution to the root of my D drive and it builds without any problem.