Hi there!
If this happens, then the tray isn't aligned correctly.
You need to play with it a little to get it working again. Nothing is broken or anything, you will get there eventually :).
The laser deck you moved (with the white slider), you might want to play with that a little. Please move it back (as you did before in the video) and then put the tray on and test again.
If you have any more question, feel free to ask! Good luck!!