[Bug] logica diferente

Tentei criar com uma logica diferente do professor, entretanto esbarrei em alguns erros:

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // TRY THIS: Try running your application with "flutter run". You'll see
        // the application has a blue toolbar. Then, without quitting the app,
        // try changing the seedColor in the colorScheme below to Colors.green
        // and then invoke "hot reload" (save your changes or press the "hot
        // reload" button in a Flutter-supported IDE, or press "r" if you used
        // the command line to start the app).
        //
        // Notice that the counter didn't reset back to zero; the application
        // state is not lost during the reload. To reset the state, use hot
        // restart instead.
        //
        // This works for code too, not just values: Most code changes can be
        // tested with just a hot reload.
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: const Text(
            "Tarefas", 
            style: TextStyle(
                color: Colors.cyan,
                fontFamily: "times new roman", 
                fontSize: 30
                  ),
                ),
            ),
        body: Column(
          mainAxisAlignment: MainAxisAlignment.start,
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            Stack(
              children: [
                Container(color: Colors.blue,height: 100, width: 100),
                Container(color: Colors.white12,height: 50, width: 50)
              ],
            )
          ],

        ),
        floatingActionButton:FloatingActionButton(onPressed: (){print("oi");},)
      )
    );
  }
}

Minha ideia era fazer uma coluna no home, alinhado no eixo principal pelo inicio, e no segundario no centro, e todos os filhos dessa coluna seriam stacks, entretanto eles nao estao indo para o centro no eixo segundario