I'm sure the code is much explanatory itself. Here is the solution. This would prevent the form being submitted the usual way while still the client validation is done.

<?php $form = ActiveForm::begin([
        'action' => ['create'],
        'enableAjaxValidation' = false,
        'enableClientValidation' = true,
        'beforeSubmit' => "function(form) {
        
                if($(form).find('.has-error').length) {
                        return false;
                }
                
                $.ajax({
                        url: form.attr('action'),
                        type: 'post',
                        data: form.serialize(),
                        success: function(data) {
                                // do something ...
                        }
                });
                
                return false;
        }",
]); ?>

This old method does not work anymore. Yii2 was updated lately to support more elegant way of handling JavaScript events. Please refer to:
https://github.com/yiisoft/yii2/pull/4955
https://github.com/yiisoft/yii2/issues/1350

Following code shows how you can use JS events to handle the same situation above.

Your php code remains same:

<?php $form = ActiveForm::begin([
        'id' => 'login-form',
        'action' => ['create'],
        'enableAjaxValidation' = false,
        'enableClientValidation' = true,
        
]); ?>

But the JavaScript part goes to a separate .js file:

$(document).ready(
        $('#login-form').on('beforeSubmit', function(event, jqXHR, settings) {
                var form = $(this);
                if(form.find('.has-error').length) {
                        return false;
                }
                
                $.ajax({
                        url: form.attr('action'),
                        type: 'post',
                        data: form.serialize(),
                        success: function(data) {
                                // do something ...
                        }
                });
                
                return false;
        }),
);