The first one is okay, if not a little awkward... The second one needs to be changed... "wish" should be something like "expectations" to make sense... I would just say "I can't do that." or "Nope."

Hello Pelin, Wishes are granted, so the first sentence "I can't grant your wish" is correct. Regards, airabeth

You asked me something, but I can't do it "You asked me to do something, but I can't do it" is much better. wishes are granted so the second sentence is correct. However a wish is not met. You can meet a demand, but not a wish.