How do I unsubscribe my Plus account?
Unsubscribing Duolingo Plus is simply impossible in my Apple Store. How will Duolingo let me unsubscribe?
This is the link to the official help text
If you don't receive a solution in the troubleshooting forum, then ask Duolingo's staff via the "email support for Plus users" in your account
I had similar problems with the Android app store.
I logged into the store in a browser after having tried to cancel my trial subscription in the app store app several times and that worked...