What does it mean if my calendar ID is invalid?
Boomerang's scheduling features require access to your Google calendars. When you configured your Bookable Schedule, you told Boomerang to put events booked for that schedule on a particular calendar (the Add to Calendar drop-down in Bookable Schedule setup).
If you've received an email that says your Google calendar is invalid or inaccessible, that means that the calendar you selected is no longer available. When this happens, Boomerang can't read this calendar or add events to it any more, and your recipients might encounter errors when viewing your schedule or when booking a time slot.
To resolve this situation, go to the Boomerang menu in Gmail, then select Bookable Schedule.
Open the affected Bookable Schedule. In the Meeting Details section, click the Add to Calendar drop-down list, and select a valid calendar. If the calendar you need isn't in the drop-down, you'll need to go to Google Calendar and either create the calendar (if it was deleted) or resolve any issue you have accessing it (if it's not a calendar you own).
Once the calendar is available in the drop-down list, you can select it and save the Bookable Schedule. This should resolve the issue described in the email, and your recipients will again be able to book meetings with you through Boomerang.