Booking Flights

I have bought a boeing 737 for my route from Newcastle to London Heathrow and this schedule is on as booked and passengers can book for this flight but I don’t know how I did it. Now I am trying to do the same for my route from Newcastle to Belfast with a different plane and I don’t know how to book these flights and it is not letting passengers book tickets for this flight. Someone please help! I am new to this simulation so I might just be not doing something right but I would appreciate if someone with more experience could help me!

thank you.

Can you maybe upload a screenshot?