Code for hooking into other plugins not working

After publishing the plugin I realized the slug has been changed so the code in the reade is incorrect. I pushed the new readme but for the meantime till it is published here is the correct code:

/wp-content/themes/Your(Child)Theme/functions.php

/* QR-Code in invoices */
add_action( 'wpo_wcpdf_after_order_details', 'wpo_wcpdf_qr_code', 10, 2 );
function wpo_wcpdf_qr_code ($document_type, $order) {
	require_once WP_PLUGIN_DIR . '/mxp-sepa-qr-code-addon-for-woocommerce/muxp-sepaqr.php';
    $muxp_order = wc_get_order( $order);
	$order_id  = $order->get_id();
 	if ( !empty($muxp_order->get_total()) && (float)$order->get_total() > 0 ) {
		echo '<h1>QR-Code for your online banking app<h1>';
		echo '<img class="bacs-qrcode" src="' . muxp_get_qrcode($order->get_total(), $order_id) . '" alt="qr-code"></p>';
	} 
}