Need to modify the filters.php file ,here : plugins\tutor\templates\course-filter\filters.php ,added a line in 12 and modified code 58-64

filters.php

<?php
$filter_object = new \TUTOR\Course_Filter();
// $filter_levels = array(
//     'beginner' => __('Beginner', 'tutor'),
//     'intermediate' => __('Intermediate', 'tutor'),
//     'expert' => __('Expert', 'tutor')
// );
$filter_prices = array(
    'free' => __('Free', 'tutor'),
    'paid' => __('Paid', 'tutor'),
);
$course_levels      = tutor_utils()->course_levels();
$supported_filters = tutor_utils()->get_option('supported_course_filters', array());
$supported_filters = array_keys($supported_filters);
?>
<form>
    <?php do_action('tutor_course_filter/before'); ?>
    <?php
    if (in_array('search', $supported_filters)) {
    ?>
        <div class="tutor-course-search-field">
            <input type="text" name="keyword" placeholder="<?php _e('Search...'); ?>" />
            <i class="tutor-icon-magnifying-glass-1"></i>
        </div>
    <?php
    }
    ?>
    <div>
        <?php
        if (in_array('category', $supported_filters)) {
        ?>
            <div>
                <h4><?php _e('Category', 'tutor'); ?></h4>
                <?php $filter_object->render_terms('category'); ?>
            </div>
        <?php
        }

        if (in_array('tag', $supported_filters)) {
        ?>
            <div>
                <h4><?php _e('Tag', 'tutor'); ?></h4>
                <?php $filter_object->render_terms('tag'); ?>
            </div>
        <?php
        }
        ?>
    </div>
    <div>
        <?php
        if (in_array('difficulty_level', $supported_filters)) {
        ?>
            <div>
                <h4><?php _e('Level', 'tutor'); ?></h4>
                <?php


                foreach ($course_levels as  $value => $title) {
                    if ($key == 'all_levels') continue; ?>

                    <label for="<?php echo esc_attr($key); ?>">
                        <input type="checkbox" name="tutor-course-filter-level" value="<?php echo $value; ?>" />&nbsp;
                        <?php echo $title; ?>
                    </label>

                <?php
                }
                ?>
            </div>
        <?php
        }


        $is_membership = get_tutor_option('monetize_by') == 'pmpro' && tutils()->has_pmpro();
        if (!$is_membership && in_array('price_type', $supported_filters)) {
        ?>
            <div>
                <h4><?php _e('Price', 'tutor'); ?></h4>
                <?php
                foreach ($filter_prices as $value => $title) {
                ?>
                    <label>
                        <input type="checkbox" name="tutor-course-filter-price" value="<?php echo $value; ?>" />&nbsp;
                        <?php echo $title; ?>
                    </label>
                <?php
                }
                ?>
            </div>
        <?php
        }
        ?>
    </div>
    <div class="tutor-clear-all-filter">
        <a href="#" onclick="window.location.reload()">
            <i class="tutor-icon-cross"></i> Clear All Filter
        </a>
    </div>
    <?php do_action('tutor_course_filter/after'); ?>
</form>