I created the client and site accounts which is shown under Site and not reseller or Client. Trash was also emptied.
Anyway, I have deleted the whole account again and and tried again. And I make sure emptied the web prefix field under: Management > SystemConfig > Settings on the tab ISP Manager. Now, I have create a client shown under Client and website under Site. Same error message "The username is already in use".
Also, I try to create the db, and this time database is web3_db1 and Database User: web3_u1.