How do i batch process denoise in lightroom classic (prev "correct" reply does not work for me

Or you can apply Denoise in Library via Quick Develop using a Preset

You will likely have to adjust Denoise amount photo by photo.

“Can’t get it to work” is somewhat vague. It doesn’t tell us the steps you followed or what you saw on the screen to indicate it didn’t work.

I get it to work via these steps:

1. In the LrC grid, select the desired photos to be denoised.
2. Switch to the Develop Module, turn on AutoSync. 
3. Perform AI Denoise on the most selected photo; LrC automatically performs AI Denoise on all selected photos.
4. Turn off AutoSync when finished.

Yes, big time saver. You can adjust the denoise level simply by dragging the AI Denoise slider after AI Denoise finishes, rather than the old method which required you to run AI Denoise a 2nd time to adjust the AI Denoise strength.