$wpdb error (Call to a member function insert() on a non-object)

$wpdb is a global variable. You have to take it into your function’s scope first …

global $wpdb;
$wpdb->insert();

… or access it per $GLOBALS

$GLOBALS['wpdb']->insert();

And I would use always lowercase keys; this is just a useful convention.

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