[Opserty]

Nulled files are ones which are edited (i.e. nulled) to allow people to use the software(/files) for free. vBulletin files, amongst other things, always have the license number displayed at the top of the PHP file. According to post #5 the License number is not there, as such it indicates that the file you added as an attachment was not downloaded from vBulletin.com and as such is not a legal vBulletin PHP File. (Basically your License may be legal but the file is not)

Maybe you uploaded the wrong file from the wrong folder or something, however what they are trying to say is that you need to ensure that you upload a fresh copy of vBulletin 3.6.8, from the vBulletin.com members area, to your server.