You are getting the error due to the Expanded widget.

Typically, Expanded widgets are placed directly inside Flex widgets.

Either remove the Expanded Widget or Wrap the Expanded Widget with a Column or Row like the following code :

class _AppBarButton extends StatelessWidget {
  final String title;
  final Function onTap;
  const _AppBarButton({
    Key key,
    this.title,
    this.onTap,
  }) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: onTap,
      child: Column(
        children: [
          Expanded(
            child: Text(
              title,
              style: const TextStyle(
                color: Colors.black,
                fontSize: 16.0,
                fontWeight: FontWeight.w600,
              ),
            ),
          ),
        ],
      ),
    );
  }
}