add_meta_box( 
        'ttl-to-edit', 
        __('Edit In Response To', 'ttl-p'), 
        array($this, 'response_to_edit'), 
        'ttl', // Custom Post Type 
        'normal', 
        'high'
);

// add_action('add_meta_boxes', array($this, 'answer_meta_action'));
add_action('admin_menu', array($this, 'answer_meta_action'));

public function answer_meta_action() {
    add_meta_box( 
        'ttl-to-edit', 
        __('Edit In Response To', 'ttl-p'), 
        array($this, 'response_to_edit'), 
        'ttl', // Custom Post Type 
        'normal', 
        'high'
    );
}

public function response_to_edit()
{
    $post_types = get_post_types();
    ?>
    <style>.clearfix {overflow: auto;}.clearfix::after {content: "";clear: both;display: table;}</style>
    <span class="tsqa_response_to_action-error" style="color:#ff0000;"></span>
    <div id="tsqa_response_to_action">
        <div style="margin: 10px 0 10px 0;" class="clearfix">
            <div class="box-section" style="width:250px;float:left;">
                <strong><label>Post Type:</label></strong> <br>
                <select name="post_type" style="width: 200px;margin-right:20px;">
                    <option value="">- Select -</option>
                    <?php foreach( $post_types as $post_type ): ?>
                        <option value="<?php echo $post_type ?>"><?php echo $post_type ?></option>
                    <?php endforeach; ?>
                </select>
            </div>
        </div>
    </div>
    <?php
}

CPT Register:

add_action('init', array($this, 'init'));
public function init() {


    $labels = array(
        'name' => _x( 'Question', 'Post Type General Name', '' ),
        ....
    );



    $args = array(
        'labels'             => $labels,
        'description'        => __( 'Questions', '' ),
        'public'             => false,
        'publicly_queryable' => false,
        'show_ui'            => true,
        'show_in_menu'       => true,
        'menu_position'      => 5,
        'menu_icon'          => 'dashicons-format-status',
        'query_var'          => true,
        'rewrite'            => array('slug'=>'question'),
        'capability_type'    => 'post',
        'capabilities'       => array(
            'create_posts'       => 'do_not_allow', 
        ), 
        'map_meta_cap'       => true,
        'has_archive'        => 'questions',
        'hierarchical'       => false,
        'menu_position'      => null,
        'supports'           => array( 'title', 'editor' ), 
    );


    register_post_type( 'ttl', $args );
    flush_rewrite_rules();
}