使用这种方法，

Image(
                image: const AssetImage("assets/bg.png"),
                height: 340,
                width: 340,
                  ),

完整代码：

import 'package:flutter/material.dart';

class LandingScreen extends StatelessWidget {
  const LandingScreen({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    return Scaffold(
      body: SafeArea(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          children: const [
            SizedBox(height: 50),
            Text(
              'Welcome to app',
              style: TextStyle(
                fontSize: 33,
                fontWeight: FontWeight.w600,
              ),
            ),
            SizedBox(height: size.height / 9),
            Image(
                image: const AssetImage("assets/bg.png"),
                height: 340,
                width: 340,
             ),
          ],
        ),
      ),
    );
  }
}

出现此问题是因为您在children: const [上使用const。
但是你是在运行时读取size.height，所以它不可能是const。
你能做到

class LandingScreen extends StatelessWidget {
  const LandingScreen({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final size = MediaQuery.of(context).size;
    return Scaffold(
      body: SafeArea(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          children: [
            const SizedBox(height: 50),
            const Text(
              'Welcome to app',
              style: TextStyle(
                fontSize: 33,
                fontWeight: FontWeight.w600,
              ),
            ),
            SizedBox(height: size.height / 9),
            Image.asset(
              'assets/bg.png',
              height: 340,
              width: 340,
            ),
          ],
        ),
      ),
    );
  }
}

您可以选中"const" and "final" keywords in Dart?