def METHOD_NAME(package_command, first_app):
    """Unknown/unsupported packaging formats raise an error."""
    # Set the packaging format
    first_app.packaging_format = "unknown"
    # Mock the actual packaging call
    package_command._package_deb = mock.MagicMock()
    # Package the app
    with pytest.raises(
        BriefcaseCommandError,
        match=r"Briefcase doesn't currently know how to build system packages in UNKNOWN format.",
    ):
        package_command.package_app(first_app)