Error Invalid option ‘6’ for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default
Pay attention to compiler “type” in the Web.Config file, when changing Framework version: for 4.5 and C#5 – type=”Microsoft.CSharp.CSharpCodeProvider… for 4.6 and C#6 – type=”Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35″ More info in this forum post