It is dependent upon having a spectre in your path which supports RapidIP2/3. For this you need MMSIM60 USR1 or later (I think) - but more sensibly you'd have a supported version (i.e. MMSIM61 or MMSIM62). Originally posted in cdnusers.org by adbeckett
If you don't have spectre from one of these streams in your path before the IC paths, then you'll pick up the old version of spectre which comes with the IC tools (in releases after IC5141, spectre is no longer included in the IC stream; the transition to a separate stream happened within the life of IC5141). The IC5141 version of spectre doesn't include many of the newer features of SpectreRF (such as Rapid IP2/3, Harmonic Balance, and so on).
I suspect that when you were using the older USR, you had MMSIM in your path, but when using USR5, you have this missing for some reason.