To use your Flutter App as an iOS Application, you need a macOS System. On older Versions of macOS you could create a virtual system on your Windows pc to run macOS, but with the current versions it is not possible.
You can read more about this topic on the apple Developer forum.
And there is another interesting discussion on Stack Overflow.
If you somehow manage it to build the app for iOS on your Windows Computer, you still need Xcode to sign and upload your app to the App Store.
To make it short, you need a macOS system to create an iOS app.
But this does not mean that your Flutter Project isn't able to build an iOS App. To initiate this process, it's just crucial to use a mac
If you don't know anyone who could help you out with a mac you could rent one in the cloud.