Normally when you don't give the actual model, or the testbench, or the error message, you make it quite difficult to help you (it is like you think we are psychic). However in this case, you yourself point out the problem. You are measuring the voltage of a pin, and then setting the voltage of the same pin to that value. In otherwords, your model states that:
Code:copin=V(cop);
V(cop)<+ copin;
This can be simplified to:
Code:V(cop)<+ V(cop);
The problem is that any voltage will satisfy that equation. Thus you have created a system where the solution is not unique, and simulators will not solve such systems. That is not really a problem in this case because this model makes no sense. Perhaps you can describe what you are trying to model and we can make suggestions.
-Ken