You access RVsitebuilder 6 from cPanel but see this error "RVsitebuilder is not installed. Please contact your host provider to install" instead of website editor.
This error lets you know that RVsitebuilder is not installed completely on this server. Please contact your host provider to run install RVsitebulder by this guide: https://rvsitebuilder.com/v6/installation/
And complete Setup in WHM as well.