When I tried to denoise the image, following the tutorial. There is no "noisy image" connector in compositing. I've enabled the "denoising data".
It only appears if you have denoising at render time enabled in the render settings panel. If you don't have render-time denoising enabled, there wouldn't be any difference between "noisy image" and the main "image" pass, because you haven't done anything to the main pass.
So just use the main "image" output on your Render Layers node, it's the same thing in this case.