"The project that we worked on for 6 months is now ruined. Thanks Scratch team." ——Not really! After fixing, the game can work completely now! Just walk to the end of the road of the first stage!
I tried many way to fix the bugs. Here is the solution: when the player walks to the end of the road, it disappears for about two secends to let the player fall. On this way, player can enter the second stage successfully!