I have successfully enjoyed having my Google calendar syncing with Thunderbird on four different computers (3 of them Linux, 1 Windows) for several years. Whenever I'd receive an invitation from Outlook or Google Calendar in my Gmail inbox, I could click an Accept button, and the appointment would be added to my Google calendar, and it would synchronize just fine. (There was a problem if the invitation was updated, but that's a separate issue. I was happy.) Recently, the Accept button is no longer displayed on invitation emails, and there is a message, "No writable calendars are configured for invitations, please check the calendar properties." I noticed it starting with Thunderbird 78.7.1 but cannot say whether it existed before, as I don't receive a lot of invitations. I have tried disconnecting all Thunderbird-related access from my calendar on the Google account settings, removed and re-installed Provider for Google Calendar (version 78.0.1, last updated March 1, 2021), and then re-authorizing access to the calendar through the dialogs when the calendar is added. I can see events on the Google calendar from Thunderbird, but the Accept button is still unavailable. Are there any suggestions what else I might try?