LR crashes when using selective masking Mac OS Monterey Mac Mini Late 2015

The specification of your Mac mini does not meet the mimimum requirements for masking features in LrC 12.x

Late 2014 Mac mini spec https://support.apple.com/kb/sp710?locale=en_GB

LrC 12.x mimimum requirements https://helpx.adobe.com/lightroom-classic/system-requirements.html

Note that GPU support in LrC 12.x requires a minimum of 2GB of memory, which is less than any of the integrated GPUs have access to on late 2014 Mac mini.

Most problems with AI masking are resolved by updating to the latest graphics driver for your hardware. On Mac that means updating to the latest version of Mac OS (13).  Unfortunately, your late 2014 Mac Mini is incompatible with Mac OS 13:

https://support.apple.com/en-us/HT213264

Disabling the graphics processor (GPU) won't help, since AI masking doesn't obey the setting of Preferences > Performance > Use Graphics Processor.  Bizarrely, Adobe has stubbornly resisted providing an option to disable the use of the GPU for AI masking:

https://community.adobe.com/t5/lightroom-classic-ideas/p-ai-masking-should-obey-preferences-gt-performance-gt-use-graphics-processor/idi-p/12480845

It may be that LR is crashing because you don't have enough graphics memory for People masking.  You can avoid the crash by keeping the People masking section collapsed:

https://helpx.adobe.com/camera-raw/kb/select-people-mask-crash.html

If that helps, you still won't be able to use People masking until Adobe provides an option letting you disable the use of the GPU for AI masking.

To help us understand your situation better:

- Post a screenshot of the error you are seeing.

- Do the LR menu command Help > System Info and copy/paste the entire contents here so we can see exactly which versions of hardware and software LR thinks you're running.

How much graphics memory do you have? What happens if you turn off the GPU in Preferences>Performance?