This is a permissions issue. Here are some steps to fix ownership and r/w permissions in a more secure way:
// check what groups your username belongs to
$ groups
// if your username doesn't belong to www-data then add username to group
$ sudo usermod -a -G www-data username
// exit ssh session (or close terminal window if local) to make group change stick
$ exit
// now give www-data group ownership of /var/www/html (you're not modifying the owner, just changing group ownership here)
$ sudo chgrp -R www-data /var/www/html
// give necessary r/w permissions directories and files to users who are members of www-data
$ sudo find /var/www/html -type d -exec chmod g=rwxs "{}" \;
$ sudo find /var/www/html -type f -exec chmod g=rw "{}" \;
// I like to check things before moving on
$ ls -l /var/www/html/
// you likely don't need to restart apache, but if you want to
$ sudo service apache2 restart
Apache should now have r/w access to /var/www/html
as well as any user that you shh/ftp in with that also belongs to www-data