Hello and welcome to the game!
If you want to sell a plane, just go to "Aircraft", select the plane you want to sell from the list and down there you have a button with multiple options: terminate lease, buy from lessor, sell aircraft. Select "sell aircraft" and click "Go". Or, you can press the "Details" button of the plane and then sell. Make sure that the plane is not leased! In that case, you need to terminate the lease (and will pay an early termination fee) or you can buy it from the lessor and then sell it.
Regarding your route problem, it might be from the fact that the has an incompatible schedule.
For example:
The plane flies from A to B. Then you want it to fly from Q to M, but it can't because it first needs to arrive in either Q or M (from A or B ) in order for it to be able to fly Q to M.
It might also be because the runway at either airport is too short for it to takeoff or land on it. Make sure to check the aircraft specifications and see the minimum runway length required.