Plesk installation fails on server with MySQL service: ERROR while trying to establish test connection

Symptoms

Plesk installation fails with the error:

Error message or request description : *****  problem report *****
ERROR while trying to establish test connection. If you are installing Plesk on an already configured MySQL server, you need to specify the administrator's credentials to succeed with the installation. To do this, you need to create a file - /root/.my.cnf with the 'client' section where you need to provide user and its password ("user = $admin_name" and "password = $admin_pass"). After installation is finished, the file /root/.my.cnf will be renamed to /root/.my.cnf.bak
Check the error reason(see log file: /var/log/plesk/install/plesk_12.0.18_installation.log), fix and try again
Synchronizing the Debian APT package index files.

Cause

Mysql had already been installed.

Resolution

1. Create /root/.my.cnf file and specify login and password for mysql user:

   # cat /root/.my.cnf
   [client]
   user = root
   password = root_password
   

2. Install Plesk again.