My domain example.com has to be a website and not a child domain

I have a secondary domain named example.com; my domain example.com is currently live with phpList script as well works fine too but due to email send/receive issues with my domain; I have contacted my hosting panel support team.
Then I have got reply as "example.com has to be a website and not a child domain. Please convert it to full website and create a mail child domain."
I need help for doing this since I have no idea that how can I do it?
I am really appreciating for any help to fix this issue.
Regards,