Hi, I dont have too much experience with surfacing in SW,

I was hoping someone could explain to me how I can accomplish this:
I have an imported part, with a sketch on it, that I would typically Cut Extrude- through all, However since its not available for surfaces I tried to work with the Trim surface command, but still not having much luck figuring it out. I've attached the part.

When I try to trim the surface, I select the sketch as the trim tool, and select the area which needs to be removed as "to remove" selection. But once I press the checkmark, it asks me to select surface to trim. Therefore I have no idea what's not allowing this occur.