Moved to SecurityCenter for better attention. The Community forum is for matters dealing with this website.
If they were blocked by either SiteAdvisor or the firewall you would most likely have seen a popup telling you that. It could also be your browser that is doing the blocking, or your ISP.
I just signed into sites.google.com using my gmail address and clicked the Create button and no 'alarms' went off so it may be something else, such as a popup blocker perhaps?
Or, if your have Parental Controls installed you are signed in as a restricted user, but that's only a guess.
You might want to contact Technical Support Online Chat linked via Useful Links at the top of this page.