When you create a Revit MEP family that requires a connector (most will) you need to pay attention to the direction of the arrow that appears on the connector. In the following image the arrow points down, out of a water closet fitting.
Yes the arrow is also different than the Flow Direction parameter in the properties of the connector.
If it were to point up then Revit would think that a pipe should connect from within the bowl of the water closet (toilet)...which would be wrong. You'd also likely get a message like this one.
This is trying to tell you that Revit just couldn't figure out how to get your fixture connected to the pipe you selected. Usually this message is related to the elevation of the pipe and the fixture or they are too close together for specified fittings to actually connect properly. When it comes to the pictured connector it means that the pipe is running the opposite direction AND that the elevation I used for the related pipe is too close to the same elevation.
Yes, you'll have to put on your detective hat, light your pipe and seek out the "elementary" explanation my "Dear Watson".
If you are curious the water closet fitting pictured above is part of a "rough-in" connector for a floor mounted toilet. The issue at present is that the basic plumbing fixtures that the architect uses in Revit Architecture has no connectors and that even if it did when their model is linked into RMEP they wouldn't be usable to connect pipe. Thus the notion of just the rough-in components instead of putting another toilet in the same place.
Turns out what we thought was "clever" is what Autodesk's RMEP team was thinking too and you'll find some basic connector families in the content with the release of Revit MEP 2009. Here's a screen capture of the mockup example.