Existing configuration file (./config.inc.php) is not readable.

1) Login to the server as root.

2) Go to the directory /usr/local/cpanel/base/3rdparty/phpMyAdmin.

[root@server~]# cd /usr/local/cpanel/base/3rdparty/phpMyAdmin

3) Check the ownership of the file config.inc.php. Check if the ownership of the file is assigned to root:cpanelphpmyadmin.

[root@server phpMyAdmin]# ls -l config.inc.php

-rw-r—– 1 root root 2409 May 25 11:18 config.inc.php

4) If the ownership is not proper set it to root:cpanelphpmyadmin.

[root@server phpMyAdmin]# chown root:cpanelphpmyadmin config.inc.php

5) Just restart the cpanel and mysql service on the server.

[root@server~]# /scripts/restartsrv cpanel

[root@server~]# /scripts/restartsrv mysql

This should sort the issue now.