Creating a Flat Surface on a Curved Surface?

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I need to put wholes in them to make an attachable spoiler thing. Its a curved surface and I cannot make holes in it, so I opened it

Bad thing is, it wont make a face anymore, it just makes empty squared and stuff, I clicked on the sies and everything, it wont make faces and I downloaded make face plugin, still wont work. Can someone help me? If you want to get hands on and stuff, email me for the file cause I cant send it becasue it reaches max capacity.

Plus even after you guys help me fix it, I am printing it from a 3d-printer and Its is hollow inside, my teacher told me it will print not hollow, like it will print filled in but how is it supossed to recongize that I need a hole here to attach my spoilers? Ask me if you need any more explaning, this is for my science fair and I need this done asap for expiriemenation and everything…please help me…and email me for anything, GenZod23@gmail.com

This is duplicate of above making surface error

You can use either Vertex Tools or Artisan to make a flat surface on a mesh