My app compiled fine, but when uploading to the App Store, I got this error":

Return status of iTunes Transporter was 1: ERROR ITMS-90208: "Invalid Bundle. The bundle does not support the minimum OS Version specified in the Info.plist."

A quick search on Google returns this post:

Basically, setting the version from 8 to 9 in ios/Flutter/AppFrameworkInfo.plist's MinimumOSVersion did the trick.