The image is not clear but i am assuming that the vertical 3 are guides and there are 2 horizontal sections. I think the error is due to guide 3. There is a sharp point on it. May be because of that software is not able to compute the geometry. Try to give round at that point while drawing guide 3. It may resolve the problem.
thanx, I'll try it!
It's because one of the guides (end one) extends beyond a section end point.
It's a bad way of creating the surface anyway. There are better ways
Create a surface that extends beyond and trim it to suit the final desired shape. Use curves without sharp deviations, that follow a similar general direction to each other, that are easily editable (parametric) et cetera, et cetera
For example ?