Not all airlines offer pre booked seats as an option, some may offer the service but not allow you to booked as part of your Thomas Cook holiday. You may need to book these on the airlines website, we have some FAQs to help you. If you can pre book your seats through the initial booking journey you'll see them in the flight extras section shown below.
If you've made a booking, you can see if you've booked your seats on your confirmation/invoice, it's in the flights section shown below. Don't forget if you've added pre booked seating to your flight via the airlines website it will not show on you Thomas Cook confirmation/invoice.