It’s likely you’ve run something out-of-order during the setup. Without knowing what that was, it’s hard to say if you’ll run into anything else further down the line.
Resetting Owner and Group
Anyway, to resolve your current issue just run this as your superuser (from any location) - Then continue where you left off. This will ensure both Owner and Group are correct:
sudo chown -R magento:www-data /var/www/html/
Therefore, you’re setting the Owner to “magento” and the Group as “www-data”, as laid out in the original Tutorial.