This issue was resolved thanks to this guide: 
https://help.ubuntu.com/community/ApacheMySQLPHP#Troubleshooting_Phpmyadmin_.26_mysql-workbench

try running these commands: 

sudo ln -s /etc/phpmyadmin/apache.conf /etc/apache2/conf-available/phpmyadmin.conf
sudo a2enconf phpmyadmin.conf
sudo systemctl restart apache2