this is an error that occurs if you have CSS Form Builder 2.0.0 installed when running the checkout wizard. it is fixed in the CSS Form Builder 2.0.1 Support release 2 version that is available to download from the CSS form builder download center page.
You will need to uninstall CCSS Form builder from the extension manager, then install the newer 2.0.1 support release 2 version.
then delete the existing checkout pages and re run the checkout wizard.