Why can’t I play game pass games when another account on the same system had already...

You can use your dad's subscription as long as you're using his Home console or his account is signed in. He can only set 1 console as his Home, so with a third console, you'd need another subscription.

You're looking for something called Xbox Game Pass Friends and Family, which is not available in all regions yet. It is currently supported in Colombia and Ireland. You would need your own account with Game Pass or to be signed in as your dad's account to play those Game Pass games.

Basically only your dad can roam

Unless you buy your own subscription this is regardless of platform as roaming is only a subscriber feature and home console only applies to one console regardless of console this includes ps xbox and even switch pro

Unfortunately if you already had/have a game pass subscription you’re unable to use that deal as its aimed at getting new subscribers on board. I had the exact same problem when i tried to get the deal just to play gears and didnt realise id already had
a game pass account without even realising 😂😂

Is the game pass subscription tied to the same account you are trying to play with or is there another account on the xbox that has the game pass. If it’s on another account, that would be the issue.