After you create your circle (arc 0 - 360) it will be going in the counterclockwise direction. In order to add arcs or curves to a detail they must go in a clockwise direction. When you click on the curve you will see an arrow indicating the direction. If a curve is going counter clockwise - which I suspect your’s is - hold the < shift > key down when selecting a curve to change the direction.
Another way to change the direction of a curve in a detail is to select the Reverse item in the details options… *note: You can tell a path is reversed if prefaced by “-” in the list.
Once you have the direction going right the seam allowance should work.