You have not assigned a value to a field. Is 'n' your new field?
syntactically it should be
python parser! NOT VB
def calculate(field1, field2):
if (field1 = 'MEDIA') and (field2 = 'PARCIALMENTE TRANSFORMADO'):
n = 20 # or '20'
else:
n = ????? # what do you want? "" or something else
return n
Expression box
def calculate(!CATEGORIA_IPD!, !IVR!)
you have to provide an option for the case where they are not equal to your queries. Move on to python. The function is the first 6 lines, and what goes into the expression box is line 9