Joining curved lines to make areas

I am having trouble creating a curved surface[- am attaching WeTransfer link to a model showing a display structure made of triangular prisms, and my attempt to create a curved surface that touches the structure to put a curved skin over it. I drew lines with the curve tool, copied it to make a second parallel one, then joined the corner points using the pencil tool, but can’t get it to admit it is a shape. What am I missing? https://we.tl/t-X8TSFLM1th

Depends on what you want to achieve here. SketchUp will autoform faces from any coplanar loop of lines, but it cant know what surfaces you want to create from a non coplanar set. If you truly want a single face there then one easy way is to build a shape that includes that wall, push pull then erase the unwanted bits. If you want a surface with thickness as it would be in the real world, build a single plane profile then push pull that to the height you want.

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Endless: I am trying for the co-planar set, but could not get it to behave as yours does. At least i now know your method- will work on it more- Thanks! DCL

Work on establishing a single plane cross section of the curved shape you want, then push pull the shape into 3D perpendicular to the arc