Initially, I encountered the error “the frequent flyer number is not recognized - formatting requirements.” I suspect the number is not recognized because it’s not linked to my Redisson account. So, I tried adding it to “My Profile/Redisson reward preference” on the web portal, but I couldn't select a frequent flyer program. Instead, I downloaded the Radisson mobile app, where I successfully updated my Radisson reward preference with my frequent flyer details.

After that, I attempted to transfer to AIR FRANCE/KLM on the web portal, and it worked. Give it a try and let me know if this works for you.