Hi,

What you have done is right, the error says the function that you are calling from the button is not defined in the corresponding model.

So in order to solve the issue, inherit the model and define the function there and call your api end point, if you already have done, ensure you have restarted the odoo service.

def button_posting(self):
​#do your code logic here

How To Add Buttons In Odoo || Odoo Button Types

Thanks