NameError in Project #30 Youtube video download

[liuxx437]

Error message received: Exception in Tkinter callback Traceback (most recent call last): File "C:\Users\liuw09\AppData\Local\Programs\Python\Python310\lib\tkinter_init_.py", line 1921, in call return self.func(*args) File "c:\Users\liuw09\OneDrive - Abbott\Projects\Python\python-mini-project\Youtube_video_download\main.py", line 25, in Download video = url.streams.filter(file_extension=extension).first() NameError: name 'extension' is not defined