Invalid set index 'pressed' on base: null instance with value of type 'bool'

Error in this code:
func _ready():
var checkbox = $MusicCheckBox
checkbox.pressed = true
var slider = $MusicSlider
slider.value = 1.0

It is saying that “checkbox” is a null instance. This means that there is not a node called “MusicCheckBox” that is a direct child of the node the script is on.

How can I then change the volume? :<

I don’t know what your scene tree looks like. You need the correct path to the MusicCheckBox node.