Apple Intelligence / Mac Mail : Summaries Unavailable.

Apple Intelligence and privacy on Mac

Note: 

Apple Intelligence is available in macOS 15.1 on Mac models with M1 or later, with Siri and device language set to U.S. English.

ghostliang wrote:

Mail summarisation is unavailable at this time. Try again later.

FWIW, it was a temporary issue. Working again.

Why are the mail summaries unavailable?