what is happening is that when you move the ground (Block) forward, the car moves with it. Then the car eventually falls off the ground.
you can see what is happening better if you set the Layout scale to 0.5 in the Start Of Layout event (event 1).
either make the ground invisible and stationary - and just have the scenery move past, or spawn new ground to move on to the screen.
I updated your file to do the latter...