First of all, this is the link to my last question：https://www.odoo.com/forum/help-1/question/why-i-modify-addnos-sale-models-sale-py-code-but-it-doesnt-work-175359

Following Ajeet Yadav's instruction,I modify my code.

But when i print pdf (execute this function.)

Error: num2chn() takes 1 positional arguments but 2 were given. 

How can i make it work correctly? (My odoo version is 13)

Thanks for any help in advance.