Translation:My maternal grandpa
Ok this is a bit ridiculous. "my maternal grandad" should be accepted, so should "my mother's father". Quite frankly they jumped the gun by putting that course in beta.
also: sometimes it allows you to leave out maternal -- but other times it marks you wrong -- be consistent
It IS in beta, as you said, so naturally it's not ready. Offer constructive criticism so they can improve it, or wait until it is officially released if it bothers you.