Will my account be charged?

Got a prime subscription on twitch because I thought I still had twitch prime. It said that I can subscribe for free.

So if you don't click on it, you shouldn't have to pay anything. In principle, you can always cancel current subscriptions in Twitch so that they are not automatically renewed.

No, if you have used the Twitch Prime free subscription, your account will not be charged.

So it is only charged when I press extend subscription?

Yes, that's what it looks like. In principle, you can check the status of all subscriptions on https://www.twitch.tv/...scriptions.