That is slightly weird.
It says “your next goodybag starts tomorrow”, and is quoting tomorrow’s date. But you should be able to cancel a queued goodybag up to 7.30pm today according to all the previously-published rules, and there’s still plenty of time for that.
I wouldn’t have thought that was anything to do with the billing preview though, because as far as we’re aware, that hasn’t yet started.
But is it a queued goodybag? If it’s a recurring goodybag, you (probably) need to turn off recurring goodybags at https://www.giffgaff.com/profile/payment-details first.
So unless somebody says differently, I’d suggest you contact an agent to find out why you can’t cancel your next