WordPress: Is its parent directory writable by the server? [Ubuntu]

The webserver (Apache) runs as user www-data and group www-data by default on Ubuntu so you need to give the permission to the file/folder are not owned by this user.

To make www-data the owner which makes it writeable:

chgrp www-data /var/www/html/wp-content/uploads
chmod g+w /var/www/html/wp-content/uploads


Programmer by day and mostly by night as well.