Pantryboy is right - the system won't let you add vouchers after payment.
You can add the voucher early so you don't forget, though. Do a dummy shop, get to the checkout page, add the voucher and get it accepted, then sign out without doing the payment. Then go back and change the order days later when you know what you want, the voucher will still be there, and complete the shop by paying.
I was having the same issue this evening but you can add the vouchers after the original checkout. You cant do it on the app but if you login to the desktop site then you can add any vouchers in on the left hand side of the screen then checkout again and it recalculates. Happy days, I had a small fortune in vouchers to be used