When I try to buy the game pass unlimited, it says you need a parent to buy this. We can’t...

There's no such thing as a Gold Unlimited. Did you buy Xbox Gold or Game Pass Ultimate?

Gold only allows to you play online multiplayer. Game Pass is like Netflix for games and allows you access to a library of games to play. Game Pass Ultimate includes both, If you only bought Gold, you would either need to buy Game Pass as well to access the game or buy the game.

So you can buy the cards from other third-party stores, and you just pay for the 10.99 for game pass. Instead of buying the game pass unlimited

When you say "buy Minecraft through Game Pass", do we presume that you mean you are buying Game Pass and then playing Minecraft through that rather than actually buying the game itself? In that case, if you stop buying Game Pass, then you will no longer be able to access the game unless you resubscribe to GP or you buy the game

buying on a pc as a gift to someone on an xbox

I did this with the unlimited game pass and went to buy a game and this happened

tried a different card too no joy

Yes, I can’t use my card to buy different games and PC Game Pass, and I can’t use the code to activate PC Game Pass. This is the message I most often receive when using an activation code: Check the instructions that came with your code. You might need to redeem it on a different web page or device, or you may be ineligible to redeem this code.