Hmm.  Yes, that's not very clear.  The same message is used both for struct.pack and for Struct.pack (which struct.pack is really just a convenient alias for);  it makes a bit more sense for the latter:

>>> struct.Struct('1s1s').pack('33')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: pack requires exactly 2 arguments