Valid SQL query not returning results

Here you’ve did something wrong with $wpdb->table_name. It will only return the default WordPress table names. Not custom table names. And also when you call $wpdb->prepare then pass the parameter as array. So your function will be like below-

function get_info($postid) {

    global $wpdb;
    // Here I assumed that your table name is '{$wpdb->prefix}places_locator'
    $sql = $wpdb->prepare("SELECT post_id, address, phone, email 
                            FROM {$wpdb->prefix}places_locator 
                            WHERE post_id = '%d'", 
                            array( $postid )
                        );

    $info = $wpdb->get_results($sql);

    return $info;

}

Now if you do print_r on get_info() with parameter post ID then it will return you value.

Use like below-

print_r(get_info(1)); // Here I assumed your post ID is 1