This is not quite correct. The error theorist can hold that a statement like "Murder is not wrong" is true, for they think that murder is not wrong or right.

Should that be "The error theorist can't hold that a statement like 'Murder is not wrong' is true"?

No. The error theorist may hold "murder is not wrong" and "murder is not right" to be true. Ey just has to hold "murder is wrong" and "murder is right" to be false, and if ey wants to endorse the "not" statements I guess a rule that "things don't have to be either right or wrong" must operate in the background.