$wpdb – joining shows no result

I don’t see where you define your custom table names.

Please try:

$wpdb->user_req = $wpdb->prefix . 'user_req';

and

$wpdb->user_req_admin = $wpdb->prefix . 'user_req_admin';

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)