get_template_part for template in subdirectory not working

What you’re looking for is get_stylesheet_directory().

get_template_part( get_stylesheet_directory() . '/modules/content/bevestiging.php');

This gives you the file path for the current theme.

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