For anyone who is reaching here and is not able to solve their issue, I used to make my widget horizontally center by setting both right and left to 0 like below:
Stack(
children: <Widget>[
Positioned(
top: 100,
left: 0,
right: 0,
child: Text("Search",
style: TextStyle(
color: Color(0xff757575),
fontWeight: FontWeight.w700,
fontFamily: "Roboto",
fontStyle: FontStyle.normal,
fontSize: 56.0),
textAlign: TextAlign.center),
),
]
)