The problem you're dealing with is the difference between evaluation and plotting the results of a function.
{I don't see any attempts to export the data (to outside of Mathcad).}
Call it a shame, or call it a blessing, but Mathcad is more forgiving when it comes to plotting than it is to evaluation.
When evaluating it insists on finding results, when plotting it only wants to show a picture. If there are e.g. unknown, or imaginary results for the evaluation of the function, it simply doesn't plot them, but it does plot all the other points.
In your case the error message at the bottom explains what's wrong. You probably have some if-statements in your function (can't see because you did not attach the worksheet), but they (apparently) do not cover all possibilities.
Be sure to cover them, and the evaluation should run.
If you need help with that, attach the worksheet.
Success!
Luc