When setting up new properties on Booking.com, or connecting your existing Booking.com listings to Hostfully, the system will need synchronize your bookings by getting the existing Booking.com bookings to the Hostfully calendar and inversely, pushing your existing Hostfully bookings to Booking.com.
In order to perform this calendar synchronization between your Hostfully and Booking.com calendars, you need to trigger the synchronization from the Channel Management page, specifically the Booking.com section, as below:
Note that if the status of the listing is another status than "Open/Bookable", like for example "Ready to open", pressing the synchronize link will get the listing into "Ready to open" mode, and you will need to push the link a second time to start the sync. This will force the import and export of bookings between the two systems, and synchronize them.
If is says "Undefined" it means the mapping was disconnected and you will need to reconnect.