Please note, this workflow based on new update 7.0
If you using older version, please update it. For paid versions, you can request the new update of Booking Calendar on this page.

1) Reason of issue.

Such issue possible, if you previously have deleted some booking resources at Booking > Resources page.
And such deleted booking resources was having some bookings. So now you are having in Database such lost bookings.

2) How to fix it.

• Open the Booking Listing page and click on "Reset search filter and user options to default values" button (1)
• Click on "Remove all booking resources" button in toolbar (2)
• You will see all lost bookings (3)
• Click on "Change resource" button and assign to this booking some exist booking resource.

Legacy. How to fix in old versions of Booking Calendar.

Please try to open this link:

https://server.com/wp-admin/admin.php?page=wpbc&wh_booking_datenext=1&wh_booking_dateprior=1&wh_booking_date=3&wh_trash=any&wh_modification_dateprior=1&wh_modification_date=3&wh_pay_status=all&view_mode=vm_listing&wh_booking_type

Since update 7.2.1 please use this link

https://server.com/wp-admin/admin.php?page=wpbc&wh_booking_datenext=1&wh_booking_dateprior=1&wh_booking_date=3&wh_trash=any&wh_modification_dateprior=1&wh_modification_date=3&wh_pay_status=all&view_mode=vm_listing&wh_booking_type=-999

Just replace "server.com" to your website DNS in this URL.

The trick in this link having empty "&wh_booking_type" parameter. Its will show all pending bookings, even if such bookings belonging to the booking resources that have been deleted.

Now, when you will see all such bookings, you will see near some bookings label that "Resource does not exist"
So you can assign to such booking(s) new exist booking resource, by clicking on specific "assign booking resource" button near this booking,
or if you do not need such booking, then please delete completely (even from trash) such booking.