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Cannot figure out why the spline curve in a sketch is not constrained.

I am new to On Shape. I have a document (see link below) in which Sketch 1 has a spline curve that I cannot constrain and I don't understand why. I believe it is preventing me from adding sketches to the surface of the spline.
The link is https://cad.onshape.com/documents/96b0af2ff41359629ba818ef/w/988c78731746b364b45ff626/e/8f3aa810307560a302a97cc9?renderMode=0&uiState=68f2e214488328ffa0acf90c
Thanks in advance for any help you can provide.
Answers
Hey Michael. The spline's control handles must also be constrained. Just give them a length and an angle referenced to some other constrained sketch element. - Scotty