From AMD Rewards: https://www.amdrewards.com/faq/rewardsPromotion/2
If the Retailer/Online Retailer/Vendor participates in the AMD Promotion you should have received a coupon from them to redeem at AMD REWARDS.
Did the ad where you purchased the Ryzen Processor mention anything about a Free game or AMD Promotion?
If it didn't then most likely that retail isn't participating in AMD Promotions. I would contact the Retailer's Customer Service and ask them.
From AMD Rewards Terms & Conditions:
This Campaign is open to participating partners. NOT ALL PARTINERS ARE PARTICIPATING.
Before purchasing a product, please verify that the Partner has chosen to participate in your
region for your desired eligible AMD product
