If you can't find any calendars in the calendar dropdown when editing a workflow but you know that the calendar exists, your calendar may be inactive and needs activating.
To verify that and enable the calendar(s), go to Settings > Calendars when you're in your GHL subaccount.
If you see Calendars that are Inactive, go ahead and activate them.
Then go back to the workflow, and you should be able to select the activated calendar in the trigger and/or actions.