Total price zero

-- HikaShop version -- : 6.3.0
-- Joomla version -- : 6.0.3
-- PHP version -- : 8.3

my products prices are $100
i have a discount coupn which is also $100

when i use it and my total price converts to zero. all payments methods are gone.
that is fine because there is no payment required.

but when i click complete order button, page refreshed and i didnt go to after checkout page

what should i do when custmoer have a coupon more than his total price ?