3D fit spline

Hello I want to sweep a circle along a 3D fit spline... The part stays red but I dont know why it doesn't work Can you give me some clues https://cad.onshape.com/documents/e8d2062de0ff95f77a6b4b37/w/88f7579ccec47391700eef0d/e/11c1c545df40cf3e4129269f?renderMode=0&uiState=62166a218a7f740bc888c9bb

I have just done a test to try and align a Bridging curve over a semi circle, and at no time is the bridging curve semicircular. It is always different. It seems these differences will detract from the ability to make consistent curves, edges and surfaces etc with predictable outcome. The bridging curve with a Magnitude of…