The requested URL phpmyadmin was not found on this server phpmyadmin

I believe this is because You haven't configure your phpmyadmin with the apache server well. If you installed the apache server and phpmyadmin using  (Because you can install them using source code etc.) below procedure may works for you.

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

sudo ln -s /usr/share/phpmyadmin /var/www/html/phpmyadmin

sudo service apache2 restart

now type localhost/phpmyadmin in web browser address bar

if you did everything correctly now you can see the phpmyadmin on the browser.

(Due to your installation method of apache restarting the server or other thing may different. If it is, then you have to do manually what commands meant to do (this description is believing you are very new to apache in linux :) ))