I would check to see how your WordPress install is managed. Bitnami is a package manager, which means it probably manages updates and adding/removing plugins directly.
Another solution might be running this via SSH:
sudo chown -R bitnami:daemon /opt/bitnami/apps/wordpress/htdocs
sudo chmod -R g+w /opt/bitnami/apps/wordpress/htdocs
sudo chmod 640 /opt/bitnami/apps/wordpress/htdocs/wp-config.php
This was a solution on a Bitnami Forum.