Check the project name in the error carefully. Chances are its not actually in your solution .sln ... and wouldn't build if it were. By default, VSTS will try to build all .csproj files regardless whether they are in your solution or not. Remove all .csproj files for projects you don't want to be built.
In my case. I deleted the My.Project directory (which contained the .csproj file) and the build suceeded.