Seems file permission issue.
cd wordpresssudo find . -type d -exec chmod 0755 {} \;sudo find . -type f -exec chmod 0644 {} \;
and following
define( 'FS_METHOD', 'direct' );
in wp-config.php
or
sudo chown -R www-data:www-data wordpress
From WHMPanel, Software, Multi PHP Manager, enable Enable PHP-FPM to fix file permissions issues.This prevents the popup appearing asking for FTP credentials.