InvalidOperationException: The build target does not support build appending. flutter
To summarize, replace the following lines in Assets > FlutterUnityIntegration > Editor > Build.cs.
For iOS, replace this line in DoBuildIOS() -
var options = BuildOptions.AcceptExternalModificationsToPlayer;
with
var options = BuildOptions.AllowDebugging;
For Android, replace this line in DoBuildAndroid() -
var options = BuildOptions.AcceptExternalModificationsToPlayer;
with
var options = BuildOptions.AllowDebugging;
EditorUserBuildSettings.exportAsGoogleAndroidProject = true;
On Android, that last line is important. It tells Unity to export an Android Studio project instead of an apk. Forgetting it will cause the copy step of the export script to fail.