Only list category if it has a custom field image

What do you try to achieve exactly ? Do you want to list categories that have an image custom field ?

If yes, try this :

 <?php
    $args = array(
      'orderby' => 'name',
      'order' => 'ASC'
    );
    $categories = get_categories($args);

    foreach($categories as $category) { 

        // check if image field is set on category
        if ( get_field( 'portada', 'category_' . $category->term_id ) ) {
            // display link
            echo '<a class="cover margin-ultimos" href="' . get_category_link( $category->term_id ) . '"><div id="cover-home" class="gray-shadow"><img src="'. get_field( 'portada', 'category_'.$category->term_id ). '" alt="Portada'. get_cat_name ( $category->term_id ) . '" /></div></a>';
        }

    } 

A few mistakes in your code :

• you should test if the image exists before displaying the link
• use get_field instead of the_field inside an echo
• you don’t need to get request a new category object because you are already looping them
• use get_cat_name instead of get_the_title when retrieving category title