VS2013 schema compare: "Source is unavailable" when running against SQL Server 2016 Express

In Visual Studio 2017 I had the same issue and the only solution that worked for me was:

Close Visual Studio, open it and try again :) It seems to be a caching issue. Another option is to rename the DB but that's not a nice way of workarounding this issue.

SQL Server Data Tools support for SQL Server 2016 is now GA and the latest update through Extensions and Updates in VS2013 and VS2015 will resolve this issue for you. You can also download directly from this page for VS2015, and this page for VS2013.

I had this error. Once I changed the server name from "pcname" to its IP "192.168.1.xxx" it worked.