[Update]
The first contact from AMD Rewards support was a not-so-helpful generic-like answer: "The detected hardware does not qualify for the promotion associated with this coupon code. If you received the wrong coupon code for your hardware purchase, please contact your retailer to obtain the correct one". I asked then for what hardware is this coupon valid, so I can check if I received the "wrong coupon code". In the meantime, I contacted the retailer reporting the issue. According to them, other clients were facing the same issue, and the store was contacting the AMD for further support.
I received other coupon code from the retailer, and fortunately it worked perfectly. I suspect that the problem is the same (or very similar) as the reported one from the 3800x thread. I guess that the keys valid for 3700X are not valid for the other processors in the same promotion tier and vice-versa (i.e. each processor has its own code).
I would like to thank the AMD Community for the information provided and hope that the OP has already solved the problem too!
