The assignments are based on a range of levels. If you are higher level than most people at your stage of the game or if you just got to a new world that will happen sometimes. If you are a member, get a friend to access it for you then teleport. If not, just skip the day and move on with the next assignment.
To add to what Seth said, don't worry about progress though, as skipping a day won't affect your overall daily rewards which I for one think is pretty cool.
