I downloaded your game, went to scratch.mit.edu and clicked start creating and then went File | Load from your computer. I then clicked on the green flag to start your game and maximised it. I was then in the Practice session and used the arrow keys but wasn't sure how to progress to the next level.
Thanks for teaching me a bit about scratch, first time I've looked at it.
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I see I've found that this is an issue with Scratch for some reason that the button works sometimes but not 100% which is quite annoying. Though it normally works the first time it's used. Though I do believe I have now fixed this issue you will just need to re-download the game. As I had to make some changes to the coding.