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

Update the following NuGet packages (whichever installed) to resolve the problem:

• Microsoft.CodeDom.Providers.DotNetCompilerPlatform
• Microsoft.Net.Compilers

1.Go to Project and select your Project properties.

2 Select Build and Click Advanced Button.

3.Select Language Version default.

4.Save .

https://youtu.be/IP8feQeWqkk