Yep Fusion Loft tool is finicky, Fusion does not like your segmented wave curves either, and the profiles are not able to be selected, usually means it is imported geometry, not native fusion curves.
If you fix that problem Fusion may get closer to cooperating for you....
Where the curves meet the skins you will need rails,
because the wing panel is slightly concave you will need a rail at the top and bottom of the curve you want to Loft,
The curves at the larger end go outside the skin, so that is not what you would want.
what is the end goal, cutting moulds with cnc, or 3d printing, my measurements predict the latter, print with infill, might work.
In the section view, you would have a rail similar to the underside of the skins, they would not be straight rails, in the front view.
There would be easier ways to add spars, straight ones, but you have a big job if you want to continue the wave spar,


Few clues for you....