why can't I make a digital recording of a DVD?

I'm looking for suggestions on how to make a digital recording from a digital music DVD. I have a yamaha CDR-D651 with an optical cable running to my receiver, which the DVD player is also connected. The owners manual on the recorder says I can make a 1st generation copy, but it simply will not record in digital- says "can't copy" on the menu. Any solutions or suggestions? thanks!