Sent. Thanks for that.
A video will not turn into a lesson
DodoLingo reads a video's own subtitles. A video with no subtitles at all, of any kind, cannot become a lesson, and there is nothing the app can do about that from here. Try another video from the same channel, or one you know has captions.
The video will not play
Playback goes through YouTube's own player, so DodoLingo needs a connection and cannot play anything offline. Some videos are also blocked by their owner from playing outside YouTube. When that happens the app offers to open the video in YouTube instead.
What the free app gives you
Five minutes of watching a day, measured as time actually watched, with no account and no sign-up. Everything else is the same as the paid app. Nothing is held back from the free version except the clock.
Managing or cancelling a subscription
Subscriptions are handled by Apple. On your iPhone open Settings, tap your name, then Subscriptions, and DodoLingo is in that list. Cancelling there stops the next renewal and leaves you subscribed until the period you have paid for runs out.
A subscription you already own on another device comes back through Restore purchases, at the foot of the subscription screen.
Refunds
Apple handles refunds for everything bought on the App Store. Ask for one at reportaproblem.apple.com with the Apple Account you paid with.
Deleting what the app knows about you
Your saved words, your streak and your library live on your phone. Deleting the app deletes them. There is no account to close, because there is no account.
The app also sends anonymous usage events, which carry a random id for the install and nothing that names you. That is the whole of what exists off your phone, and deleting the app stops any more of it. The privacy policy says what those events are.