AH00112: Warning: DocumentRoot [/html/www/html] does not exist

Actually, it's just a warning not an error, and you can ignore it. It's happened if you define DocumentRoot in the virtual host to not existed path. If you want to remove that warning, just find the virtualhost that point to that directory. Usually /etc/apache2/sites-enabled/000-default.conf contain like below:

<VirtualHost *:80>
        ServerAdmin webmaster@localhost
        DocumentRoot /var/www/html

        ErrorLog ${APACHE_LOG_DIR}/error.log
        CustomLog ${APACHE_LOG_DIR}/access.log combined

</VirtualHost>

just change DocumentRoot /var/www/html to the correct path, or just remove that virtualhost.