Flutter iOS Build Error Troubleshooting

I'm encountering an issue while trying to build my Flutter app for iOS. Despite having configured the necessary provisioning profiles and certificates on the Apple Developer portal and ensuring that the export method in the exportOptionsPlist is set to 'app-store', I'm still facing challenges.

The error message states: "Encountered error while creating the IPA: error: exportArchive exportOptionsPlist error for key "method" expected one {} but found app-store."

I'm seeking a Flutter and iOS expert who can help me troubleshoot this issue. The ideal freelancer for this project should have:
- Extensive experience with Flutter and iOS app development
- Proven track record of troubleshooting Xcode build issues
- Familiarity with configuring provisioning profiles and certificates on the Apple Developer portal
- Ability to correctly set and verify parameters in the exportOptionsPlist
- Experience with creating App Store IPA files

Please note that both my Xcode and Flutter are up to date.