This error usually occurs when a package is added while the Flutter application is running on the emulator. Hot reload or hot restart does not allow the package to be added to the application, so the application must be completely terminated and restarted.

The solution steps I will suggest

Completely terminate and restart the application

Before restart the app , do this two steps flutter clean -> flutter pub get

https://pub.dev/packages/flutter_secure_storage make sure you are doing right the installation of package