Have you added your Google Calendar to Thunderbird yet? Which version of Thunderbird are you using?

Version 91.2.0, but I've never been able to save appointments to anything other than the local calendar (which I removed as it was useless for me as you can't see a local calendar on any other device). My Google Calendar has been in Thunderbird for years.

I just want to save appointments that come in to the calendar of my choice.

I am no expert on calendar things, but might I suggest you remove your google calendar from Thunderbird and add it again. V91 brought a lot of changes to calendar functions and for some a lot of issues. But newly added calendars do not appear to have the issues.

My understanding is each calendar can only have one email address associated with it. When you right click the calendar in the calendar tab and select properties, is the erroring email address selected for the calendar?

Done. I'll know when I get my next invite and post the results.

Well, sort of solved. I get the same issue with no writable calendar, but if I double click the ics file, tell Windows to open in Thunderbird (one time), I then get a dialog box asking me which calendar the invite should go to.

That works, but there's one part of that missing. The dialog box doesn't ask about setting a reminder, so you then have to manually go into the calendar and the event and set the reminder. Forget that and no reminder...kind of a big deal.

If you've added a Google Calendar for the Gmail account that is receiving invites and you've selected that email account/identity in the calendar's properties, then you should not have to double-click the attached ics file. Simply accepting the invite should automatically save the invite in the calendar linked to the receiving email account. The event will not, however, get saved with a reminder. You'll have to edit the event and set your own reminder. If you wish to save a copy of the invite in a calendar without sending back a response to the sender of the invite, you can use the save a copy option instead, but you'll still have to manually edit the event to add a reminder that suits you.

The Gmail account is a backup for all my regular email address so that I can check things via my phone, if needed. TB does not check that email box. The invite comes in on to a different email address that does not have a calendar associated with it (for reasons I can't get into) Basically, if the calendar new event Reminder pull down menu was part of the result of double-clicking the ics, and it obeyed the default reminder rule, it would be golden.

See my mockup.