The error code E43 on Steam generally means your Steam account has been locked due to security issues, unauthorized access, ...
"Bohemia is yours to shape—official modding support for Kingdom Come: Deliverance 2 is now live on Steamworks!" developer ...
While the relevant Steam Support page and non-expired coupons still exist, it seems that Valve is steadily moving toward phasing them out entirely in favor of limiting promotional pricing to sales ...