1) This question is not related to this thread (other than that it concerns Prime) so it would have been a better idea to start a new discussion
2) Mathcad is not an equation editor like Mathtype, its supposed to yield results. It can't do that if you use variables you haven't defined (unless you use symbolic evaluation). So the errormessage you get is valid and I see no need to turn it off (and in fact you can't anyway).
3) To insert formulas just for reference purposes you can use the boolean equal sign. Guess this solves your problem. If not or if I misinterpreted your question, open a new thread and attach a worksheet to show what you are trying to do.