Get attachment by meta_key value

I think what you are after is get_post_meta() and not a custom query.

REASONS:

  • The results from custom fields are cached and extremely streamline when it cone to performance. You can read this answer I have done recently which will explain everything

  • You are trying to get the custom field attached to a post only in a single post page. This is what get_post_meta() was designed for

You can replace all your code with something like this: (Modified from the codex)

$key_1_value = get_post_meta( get_the_ID(), 'wpcf-legislacion-gratis', true ); 
// check if the custom field has a value 
if( ! empty( $key_1_value ) ) { 
    echo $key_1_value; 
}

EDIT

If you need to get the custom field value outside the loop like in a sidebar or function, you need to replace get_the_ID() with get_queried_object_id()

$key_1_value = get_post_meta( get_queried_object_id(), 'wpcf-legislacion-gratis', true ); 
// check if the custom field has a value 
if( ! empty( $key_1_value ) ) { 
    echo $key_1_value; 
}

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