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Are you using a Nintendo GBA cable? That message usually means a problem with the cable...if it's a Gameboy Color/older cable it won't work. Also, the wireless adapters don't work with Ruby, so if you're using those you'll need to get a link cable.
Is the cable a real one? When I went to the bahamas once I saw a lot of fake ones.
Anyway, is old? Look at it. is it broken?(rips, tears, strange cables...) If not it's possibly not connected well.
It either might not be connected properly, or it might be that there's some dust in one of the link slots/place where you put the cable in. Try blowing on them.