@MrJones
as you can see from the function parameter
Code:
reaper.RenderFileSection(string source_filename, string target_filename, number start_percent, number end_percent, number playrate)
it works by providing a file as source, not projects tracks.
If you want to learn more about ReaScript and how to read the API doc, you can go to my Learn ReaScript Tutorials series (link in my signature).