On_update function not working

I updated my ERPNext to version 8.5.4.

After the update, I wrote frappe.errprint("Hi") in the on_update function of the Sales Invoice. But, when I save a Sales Invoice, I do not see ‘Hi’ in the console. Anything I write in the on_update function is not happening.

Please help ASAP.

Share your code / script.

def on_update(self):

I just added the second line.

Are you on develop or production mode? In production you have to restart supervisor everytime you make a change.

I am on production mode.