I can't use let instead of var to create my variables

Link to the project or level: https://studio.code.org/projects/gamelab/7YyS6cyOSpD5wFVWwsFm1VkAysMORyJu_HHPPEsLJf0/view
What I expect to happen: the let keyword to create a variable
What actually happens: let throws an Error ERROR: Line: 9: SyntaxError: Unexpected token (9:4)
and that’s on a line i didn’t even write let on.
What I’ve tried: I’ve tried searching up on the Internet if let was a working keyword and what was it used for. I realized that what i did was right and i did nothing wrong. I also went to a Javascript compiler and tried to use the let keyword and it worked perfectly fine.

The Javascript interpreter that App Lab uses is based on Javascript version ES5. That means no let, no const. Only var.