#first copy file from 
cp /usr/share/phpmyadmin/config.sample.inc.php /usr/share/phpmyadmin/config.inc.php

#open the file
sudo nano /usr/share/phpmyadmin/config.inc.php
# then find   $cfg['blowfish_secret'] = '';
#set a passkey
$cfg['blowfish_secret'] = 'your_pass_key_here_xkdfsdf@kfa#fsd@sdf';