Hi FED, here's a demo video showing the steps after loading your file.

First thing is there is too much hidden - you need to select edges for your blend but you have hidden the edges.

So step 1 is to use Edit > Hide to show hidden objects.

Now there are some curves that are in the way - notice how those are separate objects of their own, I can just drag them away unlike the trimming edge of the surface. Also note that when I select them the object type indicator in the upper right corner says that it is a curve object (it will say either "Curve or Closed crv") when I select it, it does not say it is an edge.

So get those out of your way, here I just dragged them away but you can hide or delete them also.

Then you need to select the edges and run Construct > Blend.

Here's the video:



Hope this helps!

- Michael