Hello sewa-rat, when building a mission for a campaign it is necessary to set a particular squadron for the flight in which you wish to have your pilot, rather than the default nation you are going to fly for. So for all other flights for your nation you can use the default, but for your flight a specific squadron has to be set.
That way the sim knows where you want to be, ie first flight. The rank will then determine where in that particular flight you are, so if you want to lead then set the highest rank.
Wishing you all the very best, Pete.