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 referral and the welcome coupons both expire after 60 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.
Referral and welcome coupons may not be used for the purchase of a gift certificate.
Click this link to an article for details on what the coupon may be used to purchase: https://forever1.zendesk.com/hc/en-us/articles/360001872011-FOREVER-Referrals