How converter curve plate to sheet metal
Are you wanting it to end up as a curved sheet metal spike?
Draw your curved line on the top plane.
Covert it into a Base Flange/Tab.
Unfold the part, selecting the fixe face and bends to unfold.
Make the appropriate cuts.
Fold the part back up
Here is a link to tutorial if it helps.
The model "looks" like a sheet metal part, and may even be made of sheet metal, but it bends in two directions at once, so the sheet metal tools won't handle it.
You could use the Surface Flatten tool to flatten it. I think this tool is only available with certain installations of SolidWorks though (i.e. Premium).
See if it is available in your installation.