Is Postfix installed from a native OS package? If not, you’ll need to update the configuration in Webmin’s Postfix module to know where postfix lives.
You haven’t stated your OS, so I can’t say with confidence that all of the configuration files for your OS are known to be reliable.
You should also try running the postfix config command yourself to see what happens…if it’s broken, there’s nothing Webmin or Virtualmin can do about it. So what happens when you run:
postconf
If that does work, then you’ll need to tell Webmin where to find it in the Postfix module configuration.