Create new field and populate it with data from equation

Hi,
 
I'm having a bit o a trouble with a script. I want to builda script that creates a new field and insert data in it based on the data existing in one field.
I have uploaded the script.
Basicaly, I want to create a field called CUI and populate it with data from the field FIRMA using the equation @JustNumbersTrailing(FIRMA) but I keep getting the error "Error on line 18 - Automation object does not have a deault value".
 
I would appreciate some suggestions.