I have two identical ASP.Net 4.6 MVC project, project 1 is using roslyn complier within the site which is working fine.
c:\\windows\\system32\\inetsrv>C:\\
I just faced a similar issue and fixed it by doing as Huzzi suggested by adding the tags into the main web project's web.config and making sure that you have installed 'Microsoft.CodeDom.Providers.DotNetCompilerPlatform' from nuget.