It seems you have a prismatic joint placed between two frames that are already directly connected, either through a connection line or because they belong to the same body. You can easily reproduce this error in a smaller model to better understand why it happens:
As you can see the prismatic joint frames B&F are connected to something that is rigidly connected (in this case is the same frame!).
The frames of a joint needs to be part of two separated body parts (usually)
Did this happens with the resulting model imported using >>smimport ?
Or did the error appeared when you modifed the imported model?
This may be due to "bad" modeling in the CAD environment where joint may have been placed simply in order to build up something, but without thoughts for thedynamc behaviour of the systems. The best way to ensure repeatable import is to address the issue in the cad environment and test the model again after importing