Can’t pass var from php wp_ajax into ajax script : result undefined or null

thank you so much Tom and Sally.
My issue was indeed to not send registr_submit to.
In reallity I didn’t well understand ajax, now it’s good. I thought that serialize string of inputs form was enough to get the submit value. Mainly I convinced that php send to jquery/ajax while it is obviously the opposite.
So, in order :

ajaxdata.js:

$(function(){
      $('#registr_submit').on('click', function(e){
                e.preventDefault();
                var userdata = $('#my_registration_form').serialize();
            userdata += '&action=add_new_user&regsubmit=registr_submit';
        
            var username = $('#username').val();
                $.ajax({ 
                    url     : wpAjax.ajaxUrl,
                    data    : userdata,
                    type    : 'POST',
                    dataType: 'json',
                    success : function(response){
                        if(username == ''){
                           if(response.success == false) {
                            
                              $('#error-emp-name').html(response.data);
                           }
                        }
                        else{
                            if(response.success == true) {
                                $('#error-emp-name').html('Correcte');
                            }
                        }
                        
                  
                   }
               }
          });
    
      });

phpforajax.php:

wp_localize_script('ajax', 'wpAjax', array('ajaxUrl' => admin_url('admin-ajax.php')));
wp_enqueue_script('data-login', plugins_url('', LQUSER) . '/data/ajaxdata.js', array('jquery'), '1.0', true);

add_action('wp_ajax_add_new_user','add_new_user');
add_action('wp_ajax_nopriv_add_new_user','add_new_user');
function add_new_user(){
        

    function add_new_user(){
    global $wpdb;       

    if (isset($_POST['regsubmit']) && wp_verify_nonce($_POST['csrf'], 'csrf')) {
        
    $username       = $_POST['username'];
    $firstname      = $_POST['firstname'];
    $lastname       = $_POST['lastname'];
    $email          = $_POST['email'];
    $newpassword    = $_POST['newpassword'];
    $confirmpassword= $_POST['confirmpassword'];
    $roleuser="subscriber";
    
    if (empty($username)) {     
        wp_send_json_error('Username is required'); 
    }else{
        wp_send_json_success('Ok');
    }
    
}