//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';