Hi,
The error message basically speaks for itself.
The PHP "DOMDocument" EXTENSION is missing.
You need to install this extension to make the site work.
Which OS are you using? Are you using lsphp?
You may need to install this extension from source.
If you are using lsphp, you may try to install lsphpXX-xml (XX is the lsphp main version, e.g., 56, 55, etc) package to see whether it got your issue resolved.
You have to build matching php for lsws.
Switch to apache first, make sure it's working as expected,
and at the same time go to lsws WHM plugin, click "Build Matching PHP".
Make sure the php build is successful, and then switch to lsws.