Been trying to find something about this, but the solutions I found so far here do not work.
I need, in the single product page, to display the add to cart button li
I would suggest you override the WooCommerce template from the child theme, the file name is add-to-cart.php and can be found at woocommerce > loop.
At the bottom add the following code, it will be displayed only in product single.
if (is_single()) {
echo sprintf(
'%s %s',
esc_attr__('JUST', 'woocommerce'),
esc_attr($product->price)
);
}