Update compiled-languages-csharp.md

Author: felickz

troubleshooting/codeql-builds/compiled-languages-csharp.md

@@ -49,9 +49,12 @@ Ensure your required build tooling is installed your [runner](https://docs.githu
 If any custom tooling is required, consider pulling into your action via [custom script](https://docs.github.com/en/actions/using-github-hosted-runners/customizing-github-hosted-runners)
 
 
-### DotNet (.NET standard / core / )
+### DotNet (.NET standard / core )
 Using `dotnet` is best documented at: https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-net.  The [actions/setup-dotnet](https://github.com/actions/setup-dotnet) action can assist in configuring proper build tools.
 
+#### NuGet Error NU1301
+This can indicate your custom package server is not configured which may fail the `dotnet restore` command.  For private package servers you can follow the follwing guidance to add authentication to package sources: [Setting up authentication for nuget feeds](https://github.com/actions/setup-dotnet#setting-up-authentication-for-nuget-feeds)
+
 ### .NET Framework Manual Build Steps on Windows Runners
 NOTE: if you require windows OS to build, ensure you are using a windows runner.