Ubuntu provides phpMyAdmin as a package so it is very easy to get this installed on a server. The config file is stored in /etc/apache2/conf-enabled/phpmyadmin.conf.
If you are running Ubuntu 16 with PHP 7 then the PHP directives contained in this config file may not be working. This is a known bug: https://bugs.launchpad.net/ubuntu/+source/phpmyadmin/+bug/1637911.
Edit this file and change <IfModule mod_php.c> to <IfModule mod_php7.c>, restart Apache and it will work again.
It is more secure to increase the file upload limit here than globally for all PHP sites on your server.