Hello,

Try to use the below code for that, paste it into the functions.php file in your child theme.

add_action( 'wp_body_open', function() {
    echo '<!-- Google Tag Manager (noscript) -->
    <noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-TNBSNHLC"
    height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>
    <!-- End Google Tag Manager (noscript) -->';
} );

add_action(
	'wp_head',
	function() {
		echo '<!-- Google Tag Manager -->
        <script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
        new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
        j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
        'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
        })(window,document,'script','dataLayer','GTM-TNBSNHLC');</script>
        <!-- End Google Tag Manager -->';
	}
);

Kind Regards