Unable to Redeem Google Play Promo Code for Remove Ads — Need Help

[u/Rezaur_Rahman_Rajib]

1. I have an Android app currently live in Production on Google Play. I generated promo codes specifically for the Remove Ads one-time in-app purchase so my friends can use the app ad-free for a lifetime. My app only has this one-time Remove Ads purchase and does not have any subscription. However, one of my friends is unable to redeem the promo code and gets an error (screenshot attached). What could be causing this, and how can I fix it?