When running OCC (/var/www/nextcloud/occ), you must do it as the www-data user. For instance:
sudo -u www-data /usr/bin/php -f /var/www/nextcloud/occ db:add-missing-indicesGet a list of installed apps
sudo -u www-data php /var/www/nextcloud/occ app:listDisable an app
sudo -u www-data php /var/www/nextcloud/occ app:disable <app-name>