The skinning modifiers are all the buttons and sliders located around the Remesh All button that alter its behavior (such as the resolution and polish sliders). The Remesh All button itself also has 3 tinier buttons inside of it on the top-right corner (x,y,z) to control the axis of symmetry it uses.
Your popup is unfortunately covering your skinning modifiers so it is hard to tell. If I were to guess, I'd say you have a symmetry modifier enabled on the remesh button. If the model isn't symmetrical across that axis and the half it is trying to mirror over happens to be the half that doesn't have any vertices in it, the…