I was getting the same error because the subassembly was not fully constrained with mechanism joints. Once I constrained the components inside the subassembly, the error got solved.
I had the same problem with Creo 3.0.
SOLUTION FOR ME> Check the connections of each sub assembly, if at least the first part of those sub assemblies is not set to DEFAULT, then you got that error. So SET THAT THING TO DEFAULT! And then connect the remain parts.