Кажется простой ошибкой отступа, все должно быть в порядке:
class FC():
def data(self, a, b):
self.n1 = a
self.n2 = b
def add(self):
return (self.n1 + self.n2)
def pbnc(self, start, num):
pb = FC()
pb.data(start, start)
while (num > 0):
print(pb.add())
pb.data(pb.n2, pb.add())
num -= 1
'''
# Uncomment this part if you want this method outside of the class
def pbnc(start, num):
pb = FC()
pb.data(start, start)
while (num > 0):
print(pb.add())
pb.data(pb.n2, pb.add())
num -= 1
'''