The Cessna Citation Longitude has a problem of not being available in career mode. The official aircraft’s description says that it “Can be used in the following Microsoft Flight Simulator 2024 missions: VIP Charter.”
This forum is only for questions/issues related to the SDK so not the correct place. Please use the other official forums: https://forums.flightsimulator.com
See existing report for the issue you mentioned:
Citation Longitude / CRJ - only 2 missions and they pay almost nothing - User Interface & Activities - Microsoft Flight Simulator Forums