Hey it keep giving me this error if somebody can help.

Which Godot version? In Godot 3, it's deg2rad().

That's strange. This code works in Godot 4.0-beta6:

extends Node2D


func _ready() -> void:
	if not visible:
		return

	print_debug(deg_to_rad(180.0))
	print_debug(rad_to_deg(PI))

There are no errors or warnings, and the output is:

3.14159265358979
   At: res://deg_to_rad.gd:9:_ready()
180
   At: res://deg_to_rad.gd:10:_ready()