It's unfortunate that in the image you posted the error message obscures the geometry. It would be useful to see that to better assess the problem - could you post another picture showing the geometry?
In the form, you've selected curves 20 and 27. Are those the curves that make up the circle? Where does the marker mentioned in the error message appear?
you are running with a ridiculously small tolerance of 5.0000001E-11.
numerically a lot of the geometry computation is working in single precision so that with a coordinate value of0 .5 any numerical rounding error (round about 7 digit accuracy for single precision) will mean that the coordinates for computed points like intersections etc will be outside tolerance to any other point and nothing will join together within your 5E-11 limit.
do not set your initial db model size or global tolerance to such a numerically small value.
You can set the global tolerance manually under preferences / global .
