Time Synchrony and Fix Step simulation

Hello gentle:
There is a question from my side. I’d like to do the simulation with the time synchrony and fix step for the sensor synchronization. Bur there are always some error come out for my code:

The error:

The Code:
time setting:

    settings = world.get_settings()
    settings.fixed_delta_seconds = 0.025
    settings.synchronous_mode = True # Enables synchronous mode

**loop cycle:**
    while True:
        print('created %s',world.tick())
        world_snapshot = world.wait_for_tick(20)
        #world.on_tick(lambda world_snapshot: do_something(world_snapshot))
        print('localization', vehicle.get_transform())
        spectator.set_transform(carla.Transform(vehicle.get_transform().location + carla.Location(x=1.5,z=30),carla.Rotation(pitch=-90)))

Could you help to check the issue? and also there is some question:
1: what’s the world_tick’s reference clock. If the pygame not used in the process.
2: what’s the relationship between client clock system and the sever clock system.
Thanks a lot!