Update: the codes eventually started working for me. It seems to have been a propagation delay on Apple’s side. In my case, they became redeemable within about 48 hours.

Same issue here with a production offer code for a non-consumable IAP. The app is live on the App Store, the IAP is approved and available, and the offer is configured as a free offer because I want to give access to the paid non-consumable IAP for free. When redeeming the one-time-use production code through the App Store, I get the message that the offer is not available. The same happens with the full redemption URL generated by App Store Connect. This looks like a server-side issue with offer codes for non-consumable IAPs. Can Apple or anyone confirm whether this is currently working in production?