I am getting this error “Missing argument 2 for wpdb::prepare()”

Reason of issue.
This issue can appear, if you was using the old version of Booking Calendar (older than 4.1 version) and made update to the new WordPress version.
This issue was fixed since the update 4.1 of the Booking Calendar. You can check about that at the change-log here.

Solution.
You are need to update your version of Booking Calendar to the latest version.
If you are using the free version of Booking Calendar you can update your version in the WordPress > Plugins menu page.
If you are using the paid version of Booking Calendar you can request the new update at this page.