I never got Siri AI working under macOS 27 beta 1 (it kept saying things like "Something went wrong" when I'd just ask if it was now available), but that seems to have been due to my Macbook's SSD not having enough free space (sometimes down to only about 20GB before I move files off of it) for Siri AI to unpack and initialize its files, and then to index the SSD.
This afternoon before I installed beta 2, I freed up about 92GB, and monitored the progress of Siri AI's initialization using this command in the Terminal app, both before and after the beta 2 install:
log stream --predicate 'process == "mobileassetd" || process == "PrivateMLClientInferenceProvider" || process == "TGOnDeviceInferenceProviderService"' --info
...and it then looked happy, screaming through a bunch of processes related to Siri AI initialization and indexing, but after about an hour of running those processes under beta 1, a bunch of them were still happening, and the Siri AI app was still saying things like "Something went wrong" when I just ask it if it's ready yet, so I installed beta 2 to see if that would get Siri AI working, but still no luck.
I guess I have to wait some more. Apparently some other people are finding the same thing, even when Siri AI worked for them under beta 1, so Siri AI under beta 2 seems to be redoing a lot of re-initialization and indexing for its new files.