I have come across an issue when trying to create a Flutter iOS app with a watch app extension.
For testing I have simply created a default Flutter app using
<Flutter doesn't support Apple Watch apps due to the fact that it lacks Bitcode support for iOS as discussed in this GitHub issue.
You can track the state of Adding Bitcode support for iOS - Flutter GitHub issue, but currently it seems to have a low priority for the Flutter development team.
looks like apple watch development is supported in flutter now. here is an example:
https://github.com/magnatronus/flutter-watchtips