Allowing C# on Master Page and Layout Pages

1. Open web.config;
2. Find PageParserPaths node;
3. Add the following PageParserPath none so that it looks like so:
<PageParserPaths>
   <PageParserPath VirtualPath="/*" CompilationMode="Always" AllowServerSideScript="True" IncludeSubFolders="True" />
</PageParserPaths>