[SNIPPET OF CODE] For debugging cmb2 => this plugin doesn't work if another plugin with cmb2 library has already been activate. Working if the another is deactivated.

brozzme-form-cpt.php

$fcpt_posttype = 'submissions';

global $fcpt_posttype;
global $myCustomTypeOptions;


// Register Custom Post Type submissions
function brozzme_fcpt_submission_post_type() {
    
    global $fcpt_posttype;

    $capabilities = array(
        'publish_posts' => 'publish_submissions_entry',
        'edit_posts' => 'edit_submissions_entry',
        'edit_others_posts' => 'edit_others_submissions_entry',
        'delete_posts' => 'delete_submissions_entry',
        'delete_others_posts' => 'delete_others_submissions_entry',
        'read_private_posts' => 'read_private_submissions_entry',
        'edit_post' => 'edit_submissions_entry',
        'delete_post' => 'delete_submissions_entry',
        'read_post' => 'read_submissions_entry'
    );
    $labels = array(
        'name'                => _x( 'Demandes', 'Post Type General Name', 'brozzme' ),
        'singular_name'       => _x( 'Demande', 'Post Type Singular Name', 'brozzme' ),
        'menu_name'           => __( 'Messages reçus', 'brozzme' ),
        'parent_item_colon'   => __( 'Parent Item:', 'brozzme' ),
        'all_items'           => __( 'Messages', 'brozzme' ),
        'view_item'           => __( 'View Item', 'brozzme' ),
        'add_new_item'        => __( 'Add New Message', 'brozzme' ),
        'add_new'             => __( 'Add New', 'brozzme' ),
        'edit_item'           => __( 'Edit Item', 'brozzme' ),
        'update_item'         => __( 'Update Item', 'brozzme' ),
        'search_items'        => __( 'Search Item', 'brozzme' ),
        'not_found'           => __( 'Not found', 'brozzme' ),
        'not_found_in_trash'  => __( 'Not found in Trash', 'brozzme' ),
    );
    $args = array(
        'label'               => $fcpt_posttype,
        'description'         => __( 'Enregistrement des formulaires de demande de renseignements', 'brozzme' ),
        'labels'              => $labels,
        'taxonomies'          => array('post_tag'),
        'hierarchical'        => false,
        'public'              => true,
        'show_ui'             => true,
        'show_in_menu'        => true,
        'show_in_nav_menus'   => false,
        'show_in_admin_bar'   => true,
        'menu_position'		  => '5',
        'menu_icon' 	   	  => 'dashicons-id-alt',
        'can_export'          => true,
        'has_archive'         => true,
        'rewrite' => array('slug' => $fcpt_posttype),
        'exclude_from_search' => true,
        'publicly_queryable'  => true,
        'capability_type'     => 'post',
        'supports' => array('title', 'custom-fields', 'thumbnail' )
    );
    register_post_type( $fcpt_posttype, $args );


}

add_action( 'init', 'brozzme_fcpt_submission_post_type' );



add_filter('cmb2_meta_boxes', 'brozzme_form_cpt_metaboxes');
/**
 * Define the metabox and field configurations.
 *
 * @param  array $meta_boxes
 * @return array
 */
function brozzme_form_cpt_metaboxes( $meta_boxes ) {

    // Start with an underscore to hide fields from custom fields list
    $prefix = 'fcpt_';

    /**
     * Sample metabox to demonstrate each field type included
     */
    $meta_boxes['submissions'] = array(
        'id'         => 'submissions',
        'title'      => 'Messages',
        'object_types'   => array('submissions'), // Post type
        'context'    => 'normal',
        'priority'   => 'high',
        'show_names' => true, // Show field names on the left
        // 'cmb_styles' => true, // Enqueue the CMB stylesheet on the frontend
        'fields' => array(
            array(
                'name' => 'Instructions',
                'desc' => 'Set up your own display block.',
                'type' => 'title',
            ),
            array(
                'name' => 'Name',
                'desc' => 'user name',
                'id' => $prefix.'_name',
                'type' => 'text_medium'
            ),
            array(
                'name' => __( 'Last name', 'cmb2' ),
                'desc' => 'user last name',
                'id'   => $prefix. 'last_name',
                'type' => 'text_medium',
            ),
            array(
                'name' => 'Email',
                'desc' => 'user email',
                'id' => $prefix.'email',
                'type' => 'text_email'
            ),
            array(
                'name' => 'Message',
                'desc' => 'user message',
                'id' => $prefix.'message',
                'type' => 'textarea'
            ),

        )
    );

    return $meta_boxes;
}