The error is correct if you've any of the model in rolled back state. Please close assembly and open each part file to make sure that those are properly set.
BTW did you got a prompt for setting the rolled state of the part file when opening the assembly?
I've seen that warning also appear when no part was in rolled back state, so that is somewhat of a bug. We're using SW 2014. What usually helped was opening and editing the parts that you've been editing recently, and closing it without real changes. Don't just cancel the sketch mode, but accept it without changing anything.