What Gelert said is correct. That button is only active on macOS. Reason: https://www.amd.com/en/support/kb/release-notes/rn-prorender-blender-v2-2-blender-2-80-2-81 please read the release notes:
Blender 2.81 provides a denoiser node which is in fact the same library as our CPU ML denoiser, so on macOS we use this instead. Users can enable this in the View Layer properties, with the button marked “Enable Blender Denoiser”. Other OS’s use the RPR GPU denoiser (for speed), but there is no GPU AI denoiser on macOS.
If you are on windows and want to use the Intel denoiser instead of the GPU one either:
- Use CPU rendering and it will use the intel denoiser automatically
- Hook up a composite graph like gelert said and enable the needed aovs.
- Disable the AMD denoiser and run bpy.ops.rpr.add_denoiser_node() from the python console