Hi there and welcome aboard,
There could be some reasons why this happening; my guess is that you are trying to backup a copyrighted DVD movie, right? Well if you own the original, that's legal of course, but Nero won't allow you to. There are some other (even free) programs that can help you to so. My personal favourite is DVD Decrypter (http://www.dvddecrypter.com). This is a free program that allows DVDs to be copied to the harddrive and it even can write DVD ISO images back to a DVDR. So if you are doing a 1:1 copy and your destination disc can fit the amount of data the original DVD has (that is either a single layer DVDR for a DVD5 disc or a double layer DVDR for a DVD9 disc), this program is all you need.
If my guess of what you are trying to do is all wrong, just let us know what you are trying to do