Without looking at the surface it’s hard to say why you’re getting the message. My guess is that the shape of the offset surface is distorting the edges enough to cause incomplete edges or gaps. If you offset a smaller amount does it work? Look at it this way. If you have a concave surface and offset it, you can go one direction forever, but go the opposite direction, eventually the offset becomes too complex to calculate so gaps are formed. Hope that makes sense.