Hi Hamzah,
The error message about not being able to assemble usually means that your model is kinematically impossible. One or more degrees of freedom are constrained in multiple, inconsistent ways. Usually, you have a frame that is not properly oriented or properly positioned.
You have done a good job identifying a block that likely close to the frame that is in the wrong position or orientation. If you look at the diagrams in the documentation for the Rack and Pinion Constraint block, you will see how the frames need to be oriented and positioned. Compare these to the two frames connected to your Rack and Pinion Constraint block, and you will see the change you need to make. I would have attempted it on the model you attached, but the parameter file is missing.
Thanks,
--Steve
