Taxonomy linked to pages

Both your double and single quotes are not the correct kind in this block of code. Also, the taxonomy name is ‘colecciones’ with a small ‘c’, but you used ‘Coleccions’ with a capital ‘C’. The match is case sensitive. Thus your function code should be:

function show_product_colecciones(){

  $collections = wp_get_post_terms( get_the_ID(), 'colecciones' );

  $collection = array_pop($collections);

  $collectionPg = get_page_by_title ( $collection->name, 'OBJECT', 'page' );

  $collectionPgLink = get_permalink( $collectionPg->ID);

  echo "<b>COLECCIÓN: </b><a href="https://wordpress.stackexchange.com/questions/289311/{$collectionPgLink}">{$collection->name}</a>", '<br />';

}