I'm at stage three in my virus, I can view Skitters's shop, but I still can't buy anything, I get the "Skitters only sell to zombie" message. any ideas what's going on?

Have you tried refreshing after you get the error? That usually fixes it for me, it should only come up on occasion, not every time. I bought everything I needed yesterday though so I don't know if something has changed. As of yesterday said that it was because some of the servers hadn't caught up. Maybe try clearing your cache? Because they should be caught up by now