I tried to change define('_DB_PREFIX_', 'ps_'); in /config/settings.inc.php, but my settings.inc.php file is empty.
I tried set that define, is working, but where is saved my first configuration?
How can I change table prefix in Prestashop 1.7?
The new version of PrestaShop is migrating to the symfony framework. The configuration file that you should to change is located in:
rootofps/app/config/parameters.php
Hope it helps :)