Are you trying to purchase non FOREVER-branded digital art?
In order for the referral coupons to work on digital art they must be FOREVER Digital Art.
That art can be found here: https://www.forever.com/digital_art_library?utf8=%E2%9C%93&term=&facet_ids%5B%5D=156
Has your coupon expired?
The $5 referral and the $10 welcome coupons both expire after 30 days.
Are you trying to use a coupon that another person gave you?
Each coupon must be used in a single transaction and is non-transferable to other users. These coupons have no cash value and must be used at the time of purchase to redeem.