Thank you. That was it. There was an update to both the Federal and Georgia Fiduciary TT today (from version 021.000.0365 to version 021.000.0372) , and then I was able to efile.
It would have been nice if the error message has just said "wait for further TurboTax update" rather than suggesting to look elsewhere for workaround, scour the support site, or file by mail...and the statement that "Tax Year 2021 transmissions are not available" was plain incorrect. It was just that the present version was not updated enough.
