How to access a file in private folder in module

From what I understand you want to access private header files located in the engine directory?

You could try adding something like this to your project’s Build.cs (replace the path to match your case):

PrivateIncludePaths.AddRange(
            new string[] {
                Path.GetFullPath(Path.Combine(EngineDirectory, "Plugins/Runtime/Nvidia/Ansel/Source/Ansel/Private")),
            });