Hi Yitek, that coupon is meant to be used on Gunbot licenses, that is Standard, Pro or Ultimate, these ones:
https://gunbot.shop/product/gunbot-standard-edition/
https://gunbot.shop/product/gunbot-pro-edition/
https://gunbot.shop/product/gunbot-ultimate-edition/
After applying the coupon, you will see the price doesn’t change, but the coupon itself gives Premium support for free (no need to add Premium support item, just apply the coupon on one of the above items).
Hope this helps.