"Filling the bill" is an idiomatic expression that means to meet a certain requirement or need. The opposite of "filling the bill" would be to fall short of meeting a requirement. In this case, appropriate antonyms could be "inadequate," "insufficient," or "incompatible." The term "unfitting" could also be used to indicate something that does not meet the requirement or need. Another antonym could be "nonconforming," which suggests that something does not adhere to the necessary criteria. In sum, any term that implies a lack of satisfaction or suitability could be antonyms for "filling the bill.