You can argue that this argument is invalid because there is a lack of flow between the premise and the conclusion. The premise states that different cultures have different moral codes. The conclusion states that there is no objective truth in morality, it is simply a representation of some cultures beliefs. It is invalid that even though the premise that different cultures have different moral codes is true. This is because there could be hypothetically one moral code that is correct, such as one where there are ideal observers. Even though different cultures still have different moral codes in this hypothetical world, there is still one moral code that is correct.