Hi,
I'm not sure what that code is suppose to do, but it looks pretty strange:
extends Label
var score = 0
func _on_Bullet_ready():
score += 100
String(text)
text == "Score: " %score
score += 100 adds 100 to score, it seems ok, but String(text) is just casting text to string and probably it already is string and anyway you don't do anything with that cast result. Last line is also strange text == "Score: "%score that seems to be some comparison, not even assignment.
You probably need something like:
extends Label
var score = 0
func _on_Bullet_ready():
score += 100
text = "Score: %s" % score
