SKU underneath Single Product Title

sku isn’t a property of the product object, but try:

$product->get_sku();

For some themes, title.php could be overwritten by theme updates. The recommended way is to make a child theme and use a hook in functions.php.

I currently use a child theme, could you tell me the full code for the title.php file?

Also, could you elaborate on using a hook in functions.php? (a code possibly I can copy in)

This is for functions.php in your child theme:

add_action('woocommerce_single_product_summary', 'show_sku', 7);
function show_sku() {
  global $product;
  echo '<div class="sku">SKU: ' . $product->get_sku() . '</div>';
}

The top line of functions.php is:

<?php

You may have that already.

Hooks and filters are here:
https://docs.woothemes.com/wc-apidocs/hook-docs.html

A hook allows you to insert other content, and a filter allows you to edit the default content.

Thanks for then snippet Iorro ??