Hi so I don’t work for Roblox but I do know the reason why. Some websites sell gift cards that are for robux. What I am guessing you did was buy one of theses instead of buying a gift card that gives you credit. Also you do not need premium to buy a group now. To get credit you need to go to one of theses websites roblox provides. https://www.roblox.com/giftcards
A example of a site that does this is amazon that just gives you robux not the credit.