Why can't I withdraw my Twitch subscription?

Today I subscribed to a streamer with the free Twitch Prime Sub that I no longer want to support. When I try to cancel the subscription, it doesn't work. When I click on the manage subscription, the only option is "Change subscription level", but not the "Cancel subscription" option. Why and what can I do about it?

You have to wait until the subscription has expired. It takes a month. Then you can subscribe to someone else.

So it is not possible to cancel the subscription earlier?

No