If you receive “virtual.db: No such file or directory” error message while adding a domain in Plesk control panel, you need to create the virtual.db file and place it in the proper location. The complete error message looks like:
Failed domain creation: Unable to update domain data: mailmng failed: Fatal error: plesk::mail::postfix::PostfixConfigurationError(postmap:fatal:open database /var/spool/postfix/plesk/virtual.db: No such file or directory)
You need to manually fix the problem:
#Create the virtual.db file
postmap /etc/postfix/virtual
#Copy the .db file to the location you see in the error message:
cp /etc/postfix/virtual.db /var/spool/postfix/plesk/virtual.db
This will allow you to add domains on your server.