OpenCart cannot display per default a field during the checkout to give customer the possibility to add their coupon, gift voucher and/or reward points to reduce the checkout total.
This tiny extension will give you the opportunity just with a mouseclick to add all of them, or 2 or only 1 of them to the checkout process.

Define if coupon and/or gift voucher and/or reward points bos shall be displayed.
Additionally you can define the layout of them.
And on top define if they should be available for all or logged in customers only.