Which error is this?

[Angrboda]

One final note about the moral knowledge syllogism. William Lane Craig uses a version of this argument, but instead of moral knowledge he uses the term objective moral values (link). The difficulty there is assuming that it is evident that moral values exist objectively. (Craig's response is that we just know they do!) Whether moral values are objective or not is one of the primary debates in meta-ethics. Yet it is equally problematic to use the term moral knowledge. There is a group of meta-ethical views known as error theories which hold that morals and moral propositions have no truth value, which is to say that they are neither true nor false. Since knowledge in an approximate formulation is justified true belief, if a moral proposition can neither be true nor false, then you obviously can't have a true belief in the truth of a moral proposition. Thus if an error theory is true, then you can have moral values, but those values and our beliefs about them cannot be knowledge.

So neither premise is straight-forwardly evident in the syllogism about morals.