Hi Tessnim 

To add this feature the user must have access to this form or class ( account.payment.term)

if you want to add through the interface 

you can go to: 

configuration > group > Accounting & Finance > user (Billing)

 in the tab " Access Rights  "  clic Add an item and add ( account.payment.term) 

check all the boxes and then save

look this 

https://www.youtube.com/watch?v=93gp_umdriE