As Rubén Rodolfo Balderrama indicated, having the file to look at would help (see How can I attach a file to a forum post?), but in any case that error message typically appears when for example an arc is too small to allow for the other dimensions so that lines would interfere or cross. That would be the first thing I'd check.
My issue ended up being:
- It doesn't like splines (even though they are fully defined)
- It doesn't like offset fillets
In order to make it work:
- I had to replace the splines with several arcs to recreate the "S" curve.
- I had to draw one arc, offset the neighboring straight line, then draw a new arc from the old arc's center point and connect it to the newly offset line (and make it tangent). Therfore the arc was not offset, but rather it connected to an offset line and would adjust accordingly.
While Rubén Rodolfo Balderrama's solution would be in an ideal world, I couldn't shell as I was dealing with uneven wall thicknesses. Therefore I had to draw both lines.
Could you attach your file?