MariaDB 10.2 does not support CentOS6 anymore.

MariaDB dropped support for CentOS 6. There are no new rpm-packages for this OS version.

Whenever you try to upgrade MariaDB with custombuild of DirectAdmin you will see a warning:

• MariaDB 10.1 does not support CentOS6 anymore.
  
• MariaDB 10.2 does not support CentOS6 anymore.
• MariaDB 10.3 does not support CentOS6 anymore.
  

and so on. And a process of upgrade will stop.

It means can not upgrade MariaDB on CentOS 6. And here you have two options:

• migrate your data to CentOS 7 or CentOS 8
• or disable MariaDB updates in custombuild. In this case you won't be notified repeatedly on an available upgrade of the DB server.

Disable MariaDB updates in DirectAdmin

For this as root in a server console run the command:

cd /usr/local/directadmin/custombuild/
./build set mysql_inst no

You should see something like:

[root@server ~]# cd /usr/local/directadmin/custombuild
[root@server custombuild]#
[root@server custombuild]# ./build set mysql_inst no
Changed mysql_inst option from mariadb to no
[root@server custombuild]#

And this point you won't be notified on "available" updates. 

Still you might consider a migration at your earliest convenience.