I can't seem to get ML Denoiser to work.
I had the same error, feedback on my post:
Currently in 3ds max ML denoiser only works on AMD hardware.
This is a known issue in ProRender Max. i will send the info you provided to developers.
Bi-Weekly Beta builds of Radeon ProRender for 3DSMax Are you using this build? Should be agnostic to GPU.
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