Flutter align button to bottom of Drawer

You need to wrap your Align widget in Expanded.

drawer: Drawer(
  child: Column(
    mainAxisSize: MainAxisSize.max,
    children: <Widget>[
      Text('Top'),
      Expanded(
        child: Align(
          alignment: Alignment.bottomCenter,
          child: Text('Bottom'),
        ),
      ),
    ],
  ),
),