More information about exactly what you are doing would be helpful. One way that can lead to this problem is if you use something like IFOEdit to split a dual layer DVD into 2 discs and you failed to copy over a titleset that the DVD required. One possible solution for this particular problem is to use IFOEdit to split the disc in 2, copy all other titlesets over and then (if necessary to save space) use something like VOB Blanker to blank out unneeded titlesets. All titlesets will be present on both discs although some will be blank, but that should be enough to satisfy Nero.
However, other things could cause this message and without more details on what you are doing before you try to burn, I have no way of knowing if you are even splitting dual layer DVDs or not.