Add the_post_thumbnail_url to a shortcode in function.php

I think you’re just printing your function as a string, try concatonating:

add_action( 'woocommerce_product_meta_end', 'childReview' );
function childReview() {

  global $post;

  echo do_shortcode('[cl-popup size="l" title="Click!" btn_label="Headline" btn_bgcolor="#006982" btn_color="#ffffff" overlay_bgcolor="rgba(0,105,130,0.75)" title_textcolor="#006982" content_textcolor="#006982"]<img src="https://wordpress.stackexchange.com/questions/235313/" . wp_get_attachment_url ( get_post_thumbnail_id( $post->ID ) ) . '"/>
[/cl-popup]');
}