So, i need to display Flexible content in TCPDF, i have my Flexible content fields:

if( have_rows('levels') ):
         while ( have_rows('levels') ) : the_row();
    
    if( get_row_layout() == 'level_one' ):

        $sub = the_sub_field( 'subtitle_' );
        $pic = the_sub_field( 'image');
    
    endif;  endwhile;

    
    else :
        
    endif;

then when i try display this all in my TCPDF function, i have problem, my TCPDF function:

function getSiteContent($post_id, $fileName, $ls){
    require_once(dirname(dirname(__FILE__)).'/inc/TCPDF/examples/tcpdf_include.php');
    //$pdf = new TCPDF(PDF_PAGE_ORIENTATION, PDF_UNIT, PDF_PAGE_FORMAT, true, 'UTF-8', false);
    $pdf = new TCPDF('P', PDF_UNIT, 'A4', true, 'UTF-8', true);
    $pdf->SetCreator(PDF_CREATOR);
    $pdf->SetAuthor(get_bloginfo('admin_email'));
    $pdf->SetTitle('Site Content');
    $pdf->SetSubject('Site Content');
    $pdf->SetKeywords(get_bloginfo('name'));
    $pdf->setPrintHeader(false);
    $pdf->setPrintFooter(false);
    $pdf->SetDefaultMonospacedFont(PDF_FONT_MONOSPACED);
    $pdf->SetMargins(20, 20, 20);
    $pdf->setCellHeightRatio(1.5);
    $pdf->SetAutoPageBreak(true, 5);
    if(@file_exists(dirname(dirname(__FILE__)).'/inc/TCPDF/examples/lang/rus.php')) {
        require_once(dirname(dirname(__FILE__)).'/inc/TCPDF/examples/lang/rus.php');
        $pdf->setLanguageArray($l);
    }
    
    $pdf->SetFont('pfdindisplaypro', '', 12);
    $pdf->AddPage();
    
    //===background IMG===
    $pdf->Image($img_file, 0, 0, 3508, 2480, '', '', '', false, 150, '', false, false, 0);
    $pdf->SetAlpha(1);
    $pdf->setPageMark();

    $tagvs = array('h1' => array(0 => array('h' => 0, 'n' => 0), 1 => array('h' => 0, 'n' => 0)),
                    'p' => array(0 => array('h' => 0, 'n' => 0), 1 => array('h' => 0, 'n' => 0)));
    $pdf->setHtmlVSpace($tagvs);
    
    $html='  
        <div style="text-align:center; font-size:13px;">
            <h1 style="font-size:15px; letter-spacing:1px"><b>'.get_the_title($post_id).' </b></h1>;
    
    
            <h2 class="main__subtitle">'.$sub.'</h2>
            <img src ="'.$pic.'" />
            <br>
        </div>';
        
        $pdf->writeHTML($html, true, false, true, false, '');
    
        $pdf->lastPage();
        $pdf->Output($fileName, $ls);
}

nothing is displayed in PDF variables, where i'm wrong?