It is because you load it improperly.
add this to your global code first:
export(PackedScene) var Broken
then drag the scene to the editor's parameter
or
onready var Broken = load(“res://Assets/Objects/Player/Broken.tres”)
Though the first method is performant friendly.
After that, change the code to:
$SmokeParticle.process_material.set("color_ramp",Broken)