LR classic is version 12.3
The 2018 Mac mini uses an integrated GPU (Intel UHD Graphics 630) which has got access to around 1.5GB of shared system memory. The system requirements for LrC 12.x are linked to below. Note the minium GPU memory is 2GB. I suspect this is the reason Denoise doesn't work on your system.
That's probably the reason.
I just wondered that the function is even possible to select as some of the AI masking tools are not availble and grayed out due to a lack of graphics memory. So I supposed it should work as it is available.
I wrote the following in another thread, but forgot to included it here
'That being said, a more informative error message would help avoid the confusion caused by a meaningless error number.'