Sometimes, certain routes may not be operated, either because traffic rights prohibit it or the use of certain aircraft types is not allowed.
Aircraft Type Restrictions
The use of very small aircraft types is not allowed on routes between large airports since this is considered slot blocking. You can check whether a type may be used on a certain route on the information page of an aircraft type. Note that these restrictions only apply between airports with runways longer than 1000m.
Traffic Right Restrictions
Certain countries prohibit any operations between their airports and those of other countries they are in conflict with.
Flights within the same city or to very close airports that technically serve the same market are prohibited in AirlineSim.