The FeatureLine.ByName node likely returns a single Feature Line and not a list of Feature Lines. The code block with
a is basically just shorthand for the List.FirstItem node. It’s saying “get the item at index 0 of the ‘a’ list.” So then the warning is saying that it can’t get the item at that index because the input is not a list, but rather a single item.
So basically you could remove that code block completely and plug the Feature Line straight into the following nodes. But with that said, Feature Lines and Corridor Feature Lines are different, so you’ll probably get this warning:
You’ll have to use the nodes under the Land Development shelf to get the coordinate systems along the Feature Line, and then the planes you likely want will come from CoordinateSystem.ZXPlane.