I'm following the instructions for inserting a watermark here: https://www.princexml.com/doc/watermarks/#watermarks. However, when I generate my PDF, I get this error:

 warning: unknown page region '@prince-overlay'

Am I doing something wrong?

Which version of Prince are you using?

11.1. That was the issue. Upgrading to 11.2 resolved the problem. Thanks.

BTW, here's CSS for a diagonal watermark in red with less opacity:

@page {
   @prince-overlay {
      color: rgba(255,0,0,0.2);
      content: "CONFIDENTIAL";
      font-size: 40pt;
      -ms-transform: rotate(187deg); /* IE 9 */
      -webkit-transform: rotate(187deg); /* Chrome, Safari, Opera */
      transform: rotate(-30deg);
   }
}