Uncaught TypeError: Cannot read property 'add' of undefined

[Count92]

So this is where I'm getting the Cannot read property 'add' of undefined

my class extends Phaser.Scene. 

this.powerUps = this.physics.add.group();

But my config looks like this:

let config = {
    width: 256,
    height: 272,
    backgroundColor: 0x000000,
    pixelArt: true,
    physics:{
        default: 'arcade'
        arcade: {
            debug: false
        }
    },
    scene:[Scene1, Scene2]
}

var game = new Phaser.Game(config);

So now I'm officially lost. can someone please help.

game.js Scene2.js game.js

[Count92]

Fixed by Changing let to var. But I don't understand why that is the case.