In your polynomial symbolic expressions change the lower case x,y,z to upper case X,Y,Z. Mathcad seems to be getting confused with your previous use of x,y and z. The solution for Z is too big to be seen, but can be used later.
Its only the z which causes the problem. Rename it or use z:=z before the symbolic evaluation.
Nevertheless the polynomial form will not plot as desired as it returns a 6-element vector of z-values.
Furhermore, even if you try to construct a function which only return the first element of this vector, you run into troubles as of complex results which 3D plots does not like. You can take the absolute value of Z(x,y)[0 in the definition of f, but you would lose the sign that way and get additional, unwanted regions in your plot.
after doing what alan suggested, I saw the same problem as shown in your file. . I tried to use find() to solve for Z, but there is no one simple guess that the solution converge.. I guess I will just stay with parametric curve
