Problem
Blender version: 2.91.0
I was following the Blender Guru tutorial to create a doughnut in Blender. In Part 7, Level 1 there is a section dealing with denoising. You are required to use the 'Compositing' tab which should have a node called 'Render Layers' visible. Not only was that node not visible but the Shift-A command to add nodes did not allow anything to be added.
Solution
So simple, it hurts but I lost half an hour of my life trying to find it.
Click the 'Use Nodes' checkbox. Render Layers immediately appears.
