Vertical Divider not showing

The VerticalDivider need height for work correctly, use it with the SizedBox or equivalent with height param defined.

Example:

SizedBox(
   height: 40
   child: VerticalDivider(
      thickness: 2,
      width: 20,
      color: Colors.black,
   ),
)

You can also wrap Row into Container with specified height.

Wrap your Row with IntrinsicHeight,

IntrinsicHeight(
  child: Row(
    children: <Widget>[
      Text('420 Posts', style: TextStyle(color: Color(0xff666666)),),
      VerticalDivider(
        thickness: 2,
        width: 20,
        color: Colors.black,
      ),
      Text('420 Posts', style: TextStyle(color: Color(0xff666666)),)
    ],
  ),
)