Member Page feature was canceled in RVsitebuilder 6 already. Please see RVSitebuilder 6 Release Note here: https://support.rvglobalsoft.com/hc/en-us/articles/360000134654-RVsitebuilder-6-0-Release-Note
In case you want to protect somepage in your website, you can use "Leech Protect" feature in cPanel to do it.
Please see the guide here: https://documentation.cpanel.net/display/1152Docs/Leech+Protect
