Had to switch to core now I can't play online

Hi LMG012 G, my name is Courtney and I would be happy to help.

To understand the problem even more, I would like to ask you some questions so that I can provide you with better help. Did your Game Pass subscription expire before the estimated date?

And if you would be so kind as to offer me more details about the problem, I would appreciate it.

Best regards,
Courtney.

You can't. Game Pass Core (formerly Gold) or Ultimate (which includes Core) are required to play online with the exception of free-to-play games.

Me too. I just bought game pass core, but I can't play online. Is there any way to fix it?

Game Pass Core (or Ultimate which includes Core) is required to play online. If you haven't had the subscription before, then typically that would mean someone else had your console as their Home and you were using their subscription.

Core has always been needed to play it online. If you were able to play it before without Core/Ultimate, then it's possible someone else had your console set as their Home which was allowing you to share their subscription. If they moved their Home or lost their subscription, you would no longer be able to play online.