TB 74.0b1 is a beta version not intended for production, but solely for testing purposes. Add-on support for legacy add-ons has been removed in that version, only Webextensions (as in Firefox) or Mailextensions for that matter are supported from now on. Are you deliberately running the beta version? If so, you should be aware of that. https://www.thunderbird.net/en-US/thunderbird/74.0beta/releasenotes/ Otherwise you'll be better off using the release version, currently 68.5.0. https://www.thunderbird.net/ Install the release version. You'll then have to start Thunderbird from the CLI once with the --allow-downgrade option to allow the downgrade of the profile.

Can you try adding the calendar with CalDAV? http://www.ubuntubuzz.com/2017/07/how-to-setup-thunderbird-for-google-calendar-caldav-read-write-access.html