wp-config.php missing custom constant

Yes, the backup does not include wp-config.php

The concept that the plugin uses it that when you migrate from one WordPress to another WordPress site, the two sites will already be configured for the server that they are running.
Constants stored in wp-config.php usually apply for the specific server that the website is running and when the website is migrated onto a new server, these constants will cause issues.
For now, the solution is to manually add the custom constants in wp-config.php onto the new website before importing.