Looks like Samsung are tempting people to buy from them but making up rules as they go along. According to the ts&cs charging docks aren't excluded from the promotion but site will not accept code.
If you have the problem when you try to use the 50% off asseccories with your phone and you get error when you try to pay you need to buy through PC
It happened to me too, 50% off code promotion not working, every time I tried to use the code to buy with my phone S9 Plus, error appeared.
If you have this problem the only way for 50% discount code to work It's buying with the PC. With me after trying with the PC the code worked the first time and bought last week what I couldn't get through my phone.