Locate the correct path to config.inc.php:

sudo find / -type f -name config.inc.php 2>/dev/null

Run this command:

sudo chmod 644 <path of the file>/config.inc.php

Then verify:

ls -l <path of the file>/config.inc.php

Researt:

sudo /opt/lampp/lampp restart

#OR

sudo systemctl restart apache2