I have exactly the same problem. According to https://developer.apple.com/documentation/xcode/configuring-app-groups, the app group name on macOS should look like this: $(TeamIdentifierPrefix)com.example.mygroup.
This does solve the popup problem, but unfortunately creates another one for multi-platform applications. Because the Xcode UI does not allow you to create different app groups for different platforms, app group names that are valid for macOS are invalid for iOS and vice versa. I have not found a solution yet.
Topic:
Privacy & Security
SubTopic:
General
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