The following Terminal command worked for me: rm /var/folders/*/*/*/com.apple.dock.iconcache rm -r /var/folders/*/*/*/com.apple.iconservices* killall Dock

Just wanted to share my experience: Tried generating a promo code, didn't work. Tried generating again immediately after, still didn't work. Tried a third time—worked. So I guess the moral of the story is... just keep trying. App Store Connect surprises me in new ways every day.