Welcome to this forum.
1. There's one very important thing that you forgot to attach: the Mathcad worksheet. In many cases it is essential to provide help.
2. INC:=1;1200 does NOT define INC as an array. It defines a range. A range should only be used to index into an array, to iterate and for direct plotting.
In your case, you may want to define Freq as an array. Your simplest solution should be to use the logspace() function built into mathcad. Look it up in the help.
3. You are using the constant i (defined as the value for which i^2=-1). And that is OK.
But it means that your z values will be complex. mathcad doesn't like that in plots, it only plots real values. You probably want to plot the magnitude of the complex values |z|, or the real or imaginary parts of it Re(z), Im(z).
The example of the datasheet you apparently tried to follow is attached. Adapt it to your needs and don't hesitate to come back here in case of troubles, or questions....
Success!
Luc
