You can finalise a booking when the Booking Status is Checked out or Cancelled. So, when the status is checked-in or Booked, the booking is Not Finalisable.
Note that Finalising a booking (when status is Checked-out, No Show or Cancelled) will:
Release the security deposit
Post the invoice to Xero
Prevent any edits to the invoice or booking
You can finalise via the Booking Overview or All Bookings dashboard.