Usually, the reason R.O.B.E.R.T. would block a legitimate domain is if you have a blocker setup for particular types of websites. For example, facebook.com being blocked because you have the 'Social Networks' blocker active in R.O.B.E.R.T. You would need to disable the Social Networks blocker in this case, or whitelist 'facebook.com' in R.O.B.E.R.T. so that no R.O.B.E.R.T. rules are applied to it.

Another common reason is having the 'Ads+Trackers' blocker active. Many websites require ads and/or trackers to be loaded, else the website will not load. For this case again, you can either de-activate the Ads+Trackers blocker temporarily, or whitelist the relevant domains in R.O.B.E.R.T.

To whitelist, login to the My Account page, and select the R.O.B.E.R.T. tab. Then go to Custom Rules and add the domains you wish to whitelist.