Your node path is wrong, your camera node is child of another node and you need to take that into account to pass the correct node path and even if wasn’t a child of other node, you put the wrong node name (forgot the “3D” in the end).
Try:
@onready var camera: Camera3D = $Marker3D/Camera3D
You also can drag the node into the code editor and that will automactly paste the correct node path.
And last, i noticied you edited your post but you misunderstood what i said, the formatting buttuon i said is that one:
