header_image not returning header image url

As @Michael says, header_image() returns the URL, you need to echo it:

<img src="https://wordpress.stackexchange.com/questions/190727/<?php echo esc_url( header_image() ) ?>" ...

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