Can't find the denoiser in view layer I am using Cycles and yet it doesn't appear.

It doesn't work even when I'm using the denoiser node.

please any advice would be welcomed.
The option is hidden in the Viewer Layer Properties panel. There you will find the Denoising Data checkbox. (In Blender 2.9x it's under Passes → Data.) Tick the checkbox and the Render Layers node will get some extra sockets for the Denoise node.