I'm making a authentication login form then I face this error

When declaring a function or variable you have follow proper declaring method to avoid unusual issues. You have declare your function using Void the V is in capital case there but it must be in small case like void. Juts change this like :

final Void Function(

to

final void Function(

import 'package:flutter/material.dart';

void main(List<String> args) {
  runApp(MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData(),
      home: AuthScreen(),
    );
  }
}

class AuthScreen extends StatefulWidget {
  @override
  _AuthScreenState createState() => _AuthScreenState();
}

class _AuthScreenState extends State<AuthScreen> {
  void _submitAuthForm(
      String email, String password, String username, bool isLogin) {}
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Theme.of(context).primaryColor,
        body: Authform(
          _submitAuthForm,
        ));
  }
}

class Authform extends StatefulWidget {
  Authform(this.submitFN);

  final void Function(
      String email, String password, String username, bool isLogin) submitFN;

  @override
  _AuthformState createState() => _AuthformState();
}

class _AuthformState extends State<Authform> {
  final _formKey = GlobalKey<FormState>();

  var _isLogin = true;
  var _userEmail;
  var _userName;
  var _userPassword;

  void _trySubmit() {
    final isValid = _formKey.currentState!.validate();
    FocusScope.of(context).unfocus();
    if (isValid) {
      _formKey.currentState!.save();
      widget.submitFN(_userEmail, _userPassword, _userName, _isLogin);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Card(
        margin: EdgeInsets.all(20),
        child: SingleChildScrollView(
            child: Padding(
          padding: EdgeInsets.all(16),
          child: Form(
              key: _formKey,
              child: Column(
                mainAxisSize: MainAxisSize.min,
                children: [
                  TextFormField(
                    key: ValueKey('email'),
                    validator: (value) {
                      if (value!.isEmpty || !value.contains('@')) {
                        return 'please enter valid email address';
                      }
                      return null;
                    },
                    keyboardType: TextInputType.emailAddress,
                    decoration: InputDecoration(
                        icon: Icon(
                          Icons.mail,
                          color: Colors.orange,
                        ),
                        labelText: 'Email Addess'),
                    onSaved: (value) {
                      _userEmail = value;
                    },
                  ),
                  if (!_isLogin)
                    TextFormField(
                      key: ValueKey('username'),
                      validator: (value) {
                        if (value!.isEmpty || value.length < 4) {
                          return 'Username is short enter atleast 4 characters ';
                        }
                      },
                      decoration: InputDecoration(
                          icon: Icon(
                            Icons.person,
                            color: Colors.orange,
                          ),
                          labelText: 'Username'),
                      onSaved: (value) {
                        _userName `enter code here`= value;
                      },
                    ),
                  TextFormField(
                    key: ValueKey('password'),
                    validator: (value) {
                      if (value!.isEmpty || value.length < 7) {
                        return 'Password must be atleast 7 characters long';
                      }
                      return null;
                    },
                    decoration: InputDecoration(
                      icon: Icon(
                        Icons.lock,
                        color: Colors.orange,
                      ),
                      labelText: 'password',
                    ),
                    obscureText: true,
                    onSaved: (value) {
                      _userPassword = value;
                    },
                  ),
                  SizedBox(
                    height: 12,
                  ),
                  ElevatedButton(
                    onPressed: _trySubmit,
                    child: Text(_isLogin ? 'Login' : 'Signup'),
                    style: ElevatedButton.styleFrom(onPrimary: Colors.white),
                  ),
                  TextButton(
                      onPressed: () {
                        setState(() {
                          _isLogin = !_isLogin;
                        });
                      },
                      child: Text(_isLogin
                          ? 'Create new account'
                          : 'I already have a acoount'))
                ],
              )),
        )),
      ),
    );
  }
}