Zhang,
You guessed it! I just installed a new copy of Ubuntu Server 18.04 and installed iRedMail. This time I left the default path for mail. The installation worked perfectly!
When I completed my successful installation under Centos I noticed the ClamAV dat download. I never saw this during any of my three installations under Ubuntu.
I just watched more carefully during the installation. It appears the installer fails when it attempts to start the ClamAV update. Note the attached screenshot. Sorry, I couldn't copy/paste this text. I did this install through vCenter using the web client.