use_of_void_result

このようなエラーメッセージが出ます。

This expression has a type of 'void' so its value can't be used.
Try checking to see if you're using the correct API; there might be a function or call that returns void you didn't expect. Also check type parameters and variables which might also be void.

詳細は下記に書いてはありますが、わかりづらいです。
https://dart.dev/tools/diagnostic-messages#use_of_void_result

具体的にはこんなソースコードで発生します。

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    home: Home(),
  ));
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: FloatingActionButton(
        onPressed: () async {
          await _onPressed(context);
        },
      ),
    );
  }

  void _onPressed(context) async {
    await Future.delayed(Duration(seconds: 1));
    ScaffoldMessenger.of(context).showSnackBar(SnackBar(
      content: Text("hello"),
    ));
  }
}

こんな感じに修正すればOKです。

FloatingActionButton(onPressed: () => _onPressed(context)),