Macbook Pro crashes when using new AI Denoise

"I got a message saying denoise was not applied "unknown error"!"

That usually indicates a problem with the graphics driver.  On Mac, the only way to update graphics drivers is by updating Mac OS -- you're on Mac OS 11.7.6, for which Apple stopped most updates (except security updates) 1.5 years ago.  

So updating to Mac OS 13 or 12 might help by getting a newer driver. But you may be restricted by the age of your laptop.

Further your graphics hardware, NVIDIA GeForce GT 750M, is very old, launched January 2013.  The graphics manufacturers quietly stop fixing bugs in drivers for old hardware, so the updated drivers in Mac OS 12 or 13 may not fix the problem.

And ten-year-old graphics hardware often won't provide acceptable speed for Denoise.  There are many reports here of Denoise taking 5 - 20 minutes per photo on older graphics hardware!  

You may not have very much graphics memory on that older hardware -- System Info unfortunately doesn't always report that for Macs, but you can do Settings > Preferences > Performance, which can show the amount of graphics memory.  LR requires a minimum of 2GB, but reports here suggest that Denoise is very slow with less than 4GB, and Adobe recommends 8GB for best performance.

(no problem.)