Stu's approach worked for me. I was stuck on "Preparing" for three days. I verified that Apple Intelligence was downloaded under General -> Storage -> MacOS.
I noticed that the Language field in my Apple Intelligence & Siri setting was blank. When I selected English (United States), I received the following: "Settings Error, Extension process Siri exited."
I verified settings under General -> Language & Region. I have United States listed for the Region and English (US) - Primary listed for Preferred Languages. I tried rebooting several times, to no avail.
After reading Stu's post, I switched Siri Language to Mandarin - Taiwan (received the error that AI is not yet available, but at least did not get the Siri exited error). I was then able to switch language to English (United States) without the Siri error and was immediately notified that Apple Intelligence was ready to go.