How to test nested services spring boot

I have a registration service. I also use other entities in this service, such as passenger and trip. And I have a registerPassengerToTrip method inside registration service.

When I want to test this method, I get an error

...ValidationException: Passenger with id 9b81fdd1-9444-48bf-bdaf-bbec5f750959 not found

    public void whenRegisterPassengerToTrip_thenReturnPassInTrip(){
        // prepare
        UUID passengerId = UUID.randomUUID();
        UUID tripId = UUID.randomUUID();
        Integer seatNumber = 5;

        // testing
        PassInTrip passInTrip = registrationService.registerPassengerToTrip(passengerId, tripId, seatNumber);

        // validate
        verify(registrationRepository).save(any());
    }

How do I correctly add new passenger and new trip to whenRegisterPassengerToTrip_thenReturnPassInTrip method so that passengerId is not null? Does this require a mock PassengerRepository and TripRepository?