You need to ask TOPACHAT Retailer if they are participating in the AMD Promotion or not. If they are not then you won't receive a game coupon.
If they are participating in the AMD Promotion then it is TOPACHAT Retailer's responsibility to send you the Game coupon.
AMD doesn't give out any game coupons to owners of AMD Hardware participating in the AMD Promotion.
I would contact TOPACHAT Customer Service and find out why you haven't received your game coupon when the AMD Promotion was still in effect and they advertised it on their website.
It is possible that TOPACHAT exhausted the allotment of game coupons given to them thus you didn't get a game coupon.
But even if you do receive a Game coupon it would be voided since it is past the Redemption Period of June 20, 2020.
If you do finally get a Game Coupon from TOPACHAT Retailer you will need to open a AMD REWARDS SUPPORT ticket to explain why you are trying to redeem the game coupon after the cutoff date of June 20,2020 from here: https://www.amdrewards.com/support
Here is the Terms & Conditions of that specific AMD Promotion: