Hello, I am bad at using guide curves. I typically use a centerline instead, but this instance requires a guide curve. I've tried using the selection manager to choose the open-loop curves and the preview of what I want comes up, but it gives me a rebuild error and says that one of the open-loop sections doesn't coincide with the faces. I tried redrawing the sketch splines to ensure they intersect but still get the same error.

Does anyone have some suggestions on how the two cylinders could be joined?

One of them is a revolved cylinder, I wonder if that makes a difference