Hi @NickeGilbert and welcome,
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.
For more information, see How to diagnose and fix Ownerships and Permission issues
Executing commands as the correct user
Alternatively, your issue might be caused because you’re not switched to the “magento” (su magento) user before executing the aforementioned commands.