This happens more than you'd think — especially when developers maintain both an iOS and Android version. Screenshots must show the actual iOS app experience, not the Android version or a web prototype.
What Apple said
“The screenshots provided for your app appear to show a different platform's interface, contain UI elements inconsistent with iOS, or do not accurately represent the app's actual functionality on iOS devices. Screenshots must accurately represent the current user experience on the platform you are submitting for.”
App Store screenshots must show your actual iOS app on iOS device frames. Android screenshots have different status bar styles, navigation patterns, font rendering, and button styles that are identifiable. Using web app screenshots, design mockups that don't match the real app, or Android builds will trigger this rejection. Screenshots also can't show features that aren't in the current build.
While you're at it — Apple also requires these pages for every app.
Fix this rejection, then make sure you're covered on the compliance side too. Apple requires every app to link to a hosted Privacy Policy, Terms of Service, Support page, and Data Deletion page. No link means another rejection — just for a different reason.