Flatten() function of Fpdm libray gives error - FPDF-Merge Error: "Sorry but pdftk binary is not provided"

I am using FPDM library of FPDF to fill my pdf form and then trying to flatten it.

My code is :

$fields = array(
    'name'    => 'My name',
    'address' => 'My address',
    'city'    => 'My city',
    'phone'   => 'My phone number'
);

$pdf = new FPDM('template.pdf');
$pdf->Load( $fields, false ); 
$pdf->Merge();
$pdf->Flatten();
$pdf->Output();

But it gives error saying - FPDF-Merge Error: Sorry but pdftk binary is not provided .

As I am a new developer i have very less idea of existing libraries.

So Please help me to find the cause of this error and solution as well.