WordPress Mailerlite – How to include mailerlite SDK in plugin

This is really PHP issue, not specifically WP – the class is namespaced, so you need:

$mailerlite = new MailerLiteApi\MailerLite;

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