No cancel subscription button

Are you sure it isn't off the bottom of the screen? - can you scroll up?

If it's not there then according to Apple's documentation it's already been cancelled, but if you are not happy about this you should contact Support: iTunes Store - Contact Us - Apple Support

Good day,

If you are unable to cancel your iTunes Subscription regarding Duolingo, please contact Apple Support or iTunes Support :)

Contact Support and explain the situation.