How to loop through a custom field for each post, & display?

You are missing “foreach” loop. Try this way

    $latest_cpt = get_posts("post_type=event"); 
foreach ( $latest_cpt as $cpt_post ) {    
$theidone =$cpt_post->ID;
    $this_post_id =  $theidone; //get_the_ID();
    $key_2_value =  get_post_meta( $this_post_id, 'custom_select', true );
    if( ! empty( $key_2_value )) {
    $thisisworking =   $key_2_value ;
     ;}
    echo $thisisworking; }