Why my iPhone unable to update to the latest IOS

Assuming your iPhone model is a 6s (not a 6) and it is still running iOS 11 (?????, that's a VERY OLD version), try to update it via computer (if you don't have a computer borrow one from your friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider), then learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.

If you still get no help, contact Apple Support directly, using this link:--> Get Support

Regards

Giulio

The iPhone 6S can run iOS 15.5 But sometimes (on t he 16 gb version) it has a storage problem and iOS will not download over wifi. Try downloading it via iTunes/ a Mac computer instead.