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