Have you booked a flight with an airline other than TUI fly?
On selected flights with Smartlynx Airlines, Enter Air, Aegen Airlines, European Air Charter, Freebird Europe, FlyEgypt, Alba Star you can log in with your booking number and name and make seat reservations or add baggage.
Depending on which airline you are flying with, different conditions apply for web check-in, web check-in times and costs for check-in at the airport counter. For selected airlines, you can find detailed information here
or on the website of the respective airline.