Unable to access custom plugin backend

You’re using the admin_init hook to add menus – use admin_menu instead.

The init hook runs early enough for wp-admin/menu.php to add it to the sidebar menu, but not for the plugin page loading in wp-admin/admin.php

In other words, when you actually try to load the plugin page, your menu code hasn’t fired and WordPress fails with that rather generic error.

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