I open the Offline Mission List.
I select a mission.
There is nothing listed in flights.
I try clicking OK and it says it can't start until I pick a flight.
I thought this was a bug, that there were no flights, then I remembered to click download.
But nothing in the message conveyed that to me. It sent me off chasing wild geese.
So don't confuse them by telling them to select a flight when they can't; tell them the correct solution that they need to download the mission.
Since this is Offline Missions, the first validation on clicking OK should be: do they have it downloaded? If not, remind them.