In general, installing Ruby gems in Sketchup is not supported due to many of them using C bindings that need to be compiled separately. You’ll want to find a library that uses pure Ruby if possible.
proves that the gem is not designed to be used on Windows (although the underlying wkhtmltopdf has a Windows release). Afterall, Ruby is mostly used for web development and web apps are almost solely deployed on unixoid systems. The developer didn’t even care to mention system requirements.
What you can do:
request from the SketchUp team to make available a SketchUp build for Linux (at least for a Linux SDK there would be very serious use cases, since it allows .skp files to be used in web apps). Others have failed with that, and even if, barely anyone of your extension users would fulfill the requirements of your extension.
develop your extension on macOS, which is unixoid. This would restrict your extension to be solely compatible for macOS users.
use a different PDF library that can be used on Windows and macOS
figure out how to apply this hacky workaround to make pdfkit use the Windows version of wkhtmltopdf. Apparently you need to call the method wkhtmltopdf=(path). Don’t forget that this workaround must work for everyone who installs your extension.
If “Prawn::Html” is uninitialized, it means you need to initialize it. That means you need to check whether the gem (that defines it) is loaded and whether it really defines Prawn::Html and that it has not been renamed.