If you get the following error while transfering an account in cPanel/WHM

1. Login to mysql and search if a Database user really exists in the MySQL DB

SELECT user FROM mysql.user;

2. If the user exists, drop the user using

DROP user <db_username>;

3. If the user does not exists in MySQL database, then remove the conflicting Database Owner in the below files

vi /var/cpanel/databases/users.db

and

vi /etc/dbowners

Once this is done, try to transfer the account again and hopefully this time it should complete without any errors.