Add custom order meta to Order Completed email

paste the below code your active theme functions.php file

add_filter( 'woocommerce_email_order_meta_fields','custom_order_email_order_meta_data', 10, 3 );


function custom_order_email_order_meta_data( $fields, $sent_to_admin, $order ) {

$fields['wpcf-purchase-student'] = array(
    'label' => __( 'purchase amount', 'woocommerce' ),
    'value' =>  get_post_meta( $order->id, 'wpcf-purchase-student', true ) 
);
return $fields;
}