When you run a scene your script instance is created but hasn't yet entered the tree, that's why your DamageArea still has no parent (hence the get_parent() gives you a null instance).
What you need to do is either declare the variable on the ready function (wich runs right after your node and children enter the scene tree):
var knockback = get_parent().knockback
Or much easier, just declare it as an onready var, it does the same but less bloated:
onready var knockback = get_parent().knockback
Hope it helped.