When connecting to the MySQL screens in DirectAdmin, if this error appears, that would indicated that the "da_admin" user has not been setup correctly. To resolve this do the following.

1) Make sure the root mysql password works. If you know it, skip to 2). The mysql root password can be found in the /usr/local/directadmin/scripts/setup.txt if it has not been deleted. It's under the header "mysql=". If it cannot be found, then mysqld will have to be restarted with the --skip-grant-tables option:
service mysqld stop (Redhat)
mysqld_safe --skip-grant-tables &
That should start up mysql without the need for a root password. Once in, type
use mysql
UPDATE user SET password=PASSWORD('newpass') WHERE user='root';
FLUSH PRIVILEGES;
quit
That will reset the root password for you.

Type "killall -9 mysqld_safe; killall -9 mysqld" to shut down mysqld.
Start it up again with
/sbin/service mysqld start
2) Once the root mysql password is set and known, then you can begin the process of resetting the da_admin mysql user. Type:
mysql -uroot -p
Then press enter. You'll be asked for the password. Once in mysql, type:
GRANT ALL PRIVILEGES ON *.* TO da_admin@localhost IDENTIFIED BY 'newdapass' WITH GRANT OPTION;
FLUSH PRIVILEGES;
quit
That should set the password for da_admin in mysql.
3) Now we need to make sure it's setup correctly for DA to use. Edit /usr/local/directadmin/conf/mysql.conf and set
user=da_admin
passwd=newdapass
4) Test it out in DirectAdmin.

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Best Regards,
Jahangir Alam.