Yeah, need to be a little more specific about what exactly you're trying to do, and what error(s) you're seeing.
Most store-bought video DVDs are copy-protected specifically to prevent what you're trying to do, and asking about ways around it is not allowed here will get the thread locked.
If it's a data DVD, the only potential problem is that you don't have enough free space on your drive, as Nero needs to copy the original disc to a temporary file first (for a regular DVD, you need at least 5GB of free drive space; for a dual-layer, you'd need at least 9GB free).
The other thing, as CDATech notes, is that if your original is a dual-layer DVD, it can have up to 9GB of data, which won't fit onto a regular DVD; you either need a dual-layer capable burner and dual-layer blank, or you need to pare down the data from the original.