I kinda figured out what the problem is.
The save files related to the current score and best score doesn't exist and I haven't write the proper codes to make it exist yet.
I tried to write the code as such:-
if not save_score.file_exists(save_score):
create_save_score()
pass
The create_save_score() is defined like this:-
func create_save_score():
save_score.open(save_path_score, File.WRITE)
save_score.store_var(save_data)
save_score.close()
pass
The value is nil so I put an if condition in ready(), as such:-
if save_data["score"] == TYPE_NIL:
save_data["score"] = 1
Then I save it, read it, but it still return nil.
Help :/ lol