It turned out to be that I was typing the name of the tree item wrong. In my case there was a missing space between (f=2;z=10) and [1] which was difficult to see in the navigation tree. So it gave me that error because that command was doing nothing and after that, the macro was trying to use ASCII Export with another item selected ("Anchor points", as you can see in my picture). Although I managed to make it work, when CST exports the fields I only get zeros in all points. I don't know if I'm setting the parameters correctly. I also tried to export it manually with different values of Dx or Nx but I get the same zeros