There are some reasons why you can’t pre-order products for a flight. The most common ones are:
Your flight is not operated by Lufthansa oder Lufthansa Cityline.
Certain destinations are not part of the offer e.g. Tel Aviv.
You have not adhered to the prescribed period for advance bookings. You can find the period for your flight under “What is the time frame to pre-order products for my flight?”
The maximum load quantity has already been reached on a flight, so no additional products can be ordered.