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; }