Ok, so I've taken a look at it, and it looks like it works fine for me, I'm using Win 10 with the Edge Browser and this combo allows me to freely use it.
I've figured out, why your game ends when you click on an item: An click on an item instantly removes the amount of melons from your account, which means: if you press the button twice, you will lose twice the amount, this isn't the case if you use the hotkeys.
Keep an eye on the "Melons eaten per second" 'cause you should add that into your tactic to not get a game over.
I hope, I could help you, please write back, if that wasn't the case!
Have a nice day!