error 000861: Destination field is not valid:

Hi everyone,

First I would like to highlight that I´m an absolut newby with python.
I trying to use this already created python tool but keep on receiving this error message: error 000861: Destination field is not valid:

It's a nested code so would be difficult to put all the different variables but the specific part of the code that is giving this problem is this:

All the input variables are fine, the only error cames from the last one (destination field) wich it´s supposed to be an optional variable. That´s why the "", because is empty.

Any suggestions??

Hope can understand the post.
Thanks!!