Sharon Street has argued that (prominent versions of) quasi-realist expressivism ("expressivism," for short) has a knowledge problem. Others, such as Allan Gibbard and Jamie Dreier, deny this. I'm inclined to agree with Sharon on this matter, albeit for reasons somewhat different from those she offers when she states her challenge.
Stating what the alleged problem is requires a bit of set up. The first step is to get a sense of what distinguishes quasi-realist expressivism from realism. Here is one way to do so that meshes with much of what expressivists say.