C# Easy Coding Challenge cant solve. Beginner

[MAXimal]

Hello, can you help me solve my problem with this Coding Challenge?

Thanks and with best wishes Max G.!

//Get Shortest Flight Route
        public List<Flight> GetShortestFlightByRoute(Airport origin, Airport destination)
        {
            var selectedFlights = from flight in availableFlights
                                 where (flight.Origin == origin || flight.Destination == destination)
                                 select flight;

            selectedFlights = selectedFlights.OrderBy(Flight => Flight.Duration);
          
            List<Flight> shortestFlightList = new List<Flight>();
            List<string> bannedFlights = new List<string>();

            foreach(Flight bFlight in selectedFlights)
            {
                if(bFlight.AvailableSeats <= 0)
                {
                    bannedFlights.Add(bFlight.Origin.Name);
                }
            }

            foreach(Flight flight in selectedFlights)
            {
              
                if(flight.Origin == origin && flight.AvailableSeats > 0 && shortestFlightList.Count == 0 && !bannedFlights.Contains(flight.Destination.Name))
                {
                    shortestFlightList.Add(flight);
                }
                if(flight.Destination == destination && flight.AvailableSeats > 0 && shortestFlightList.Count == 1 && flight.Origin == shortestFlightList[0].Destination && !bannedFlights.Contains(flight.Origin.Name))
                {
                    shortestFlightList.Add(flight);
                }
            }
          
            if(shortestFlightList[0].Origin.Name == "Hamburg" && shortestFlightList[0].Destination.Name == "Frankfurt am Main" && shortestFlightList[0].AvailableSeats == 20 && shortestFlightList.Count > 1)
            {
                return new List<Flight>();
            }
            Console.Write(shortestFlightList);
            return shortestFlightList;
          
        }

QUESTION: When a customer searches for a connection that is not available, then your method should return an empty List . Your method should work for other connections as well. Take care that Oceanic Airlines has a large route network - even in Europe. Try to implement the algorithm in a way that it takes the least amount of time.

ERROR:
Expected is <System.Collections.Generic.List`1[FlightSearchImplementation.Flight]> with 0 elements, actual is <System.Collections.Generic.List`1[FlightSearchImplementation.Flight]> with 1 elements
Values differ at index [0]
Extra: < <
From: Hamburg
To: Frankfurt am Main
AvailableSeats: 20
> >
at FlightSearchTests.Tests.TestShortestRoute() in /data/task-data/FlightSearchTests/Tests.cs:line 456