Incorrect use of ParentData Widget. Expanded widget Must be Placed Inside the flex widget.

Expanded should only be descendant of a column, row or flex.

So, if anywhere expanded is Present in other than column , row or flex widget, try removing those widget.

For Example → Simply Using the Expanded wiget in the Conatiner can cause Error.

            Container(
              child: Text(
                "Hello",
                style: TextStyle(fontWeight: FontWeight.bold),
              ),
             child: Expanded(
                child: Text("HEY")
             ),
        ),

Try to wrap it in Column / Row or Flex Widget.

 Column(
          children: [
            Container(
              child: Text(
                "Hello",
                style: TextStyle(fontWeight: FontWeight.bold),
              ),
            ),
          Expanded(
             child: Text("Hey"),
          ),            
          ],
);

Simply wrap Expanded in the Row/Column or Flex widget and your Problem will be Solved.

Feel free to ask if you face any issue regarding this.