I created this custom form and put it in mysite/code and named it NewRegistrationForm.php

class NewRegistrationForm extends Form {

    public function __construct($controller, $name) {
        $fields = new FieldList(
            EmailField::create("Email"),
            PasswordField::create("Password")
        );
        $actions = new FieldList(FormAction::create("login")->setTitle("Log in"));
        
        parent::__construct($controller, $name, $fields, $actions);
    }
     
    public function login(array $data, Form $form) {
        // Do something with $data
        Controller::curr()->redirectBack();
    }
     
    public function forTemplate() {
        return $this->renderWith(array($this->class, 'Form'));
    }
}
	

When i did a ?flush=1, it did not flush properly and displayed this PHP error:
Fatal error: Class 'FieldList' not found in <omitted full path here>mysite/code/NewRegistrationForm.php on line 6

Am i missing something?

Are you on 3.0? FieldList was introduced in 3.0.

I'm using 2.4.5

Then make sure you read documentation for 2.4 and use 2.4 supported modules. Your NewRegistrationForm class has mostly 3.0 related code so that will need to be changed to something like

public function __construct($controller, $name) {
$fields = new FieldSet(
new EmailField("Email"),
new PasswordField("Password")
);
$actions = new FieldSet(new FormAction('login', 'Login'));

parent::__construct($controller, $name, $fields, $actions);
}

Though I wonder why you're creating a custom login form? You should use MemberAuthenicator and MemberLoginForm for handling security authentication as this will do all the work for you. Both of those classes can be altered via Object::useCustomClass() if you need to customize.