 So, sometimes ZBrush would tell you, sorry we can't dine a mesh or zero mesh or mirror, because your mesh sucks, not a problem. When this happens, the first thing I always try is go into geometry, mesh integrity, check mesh, it'll tell you what's wrong, then you just click fix mesh, you're done. Doesn't always work, but it's always worth a shot. Hope that helps, and as always, hope you have a fantastic day, and I'll see you around.