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im new to godot and i don't know the problem here can someone please can help?

Can you post the full script with the console output ?

Also this looks wrong on many levels, if you want to reference the CollisionShape2D node in your tree, you need to reference it and make it into a variable first. Here's an exemple script

Node setup :

Script :

extends CharacterBody2D

@onready var collision_shape_2D_shape = $CollisionShape2D.shape

func _crouch(delta):
	if Input.is_action_just_pressed("SHIFT") and is_on_floor():
		collision_shape_2D_shape.radius = 0.25
	else:
		collision_shape_2D_shape.radius =  0.50

    Can't really help you without seeing the node hiarchy.