The Uniden Wiki product guide for the BCD996XT says the scanner comes with a "Serial" programming cable rather than a USB (USB-1) cable. That was also the case for my BCD396XT purchased factory new. It included a serial rather than a USB programming cable. Did your BCD996XT come with a serial or USB programming cable as part of the factory packaging?
If you have a newer computer which only has USB ports, then you have a couple of choices. You can purchase a USB-1 cable made by Uniden for about 25 bucks or get a USB/Serial adaptor plug for at least have that price. Not all USB/Serial adaptors work, so do a search on this forum for recommendations. As I recall, I have read posts saying low price vendors Meritline and the MonoPrice products worked. I use a Best Buy house brand Dynex without any problems. It attaches to the serial programming cable that came with my BCD396T and has also worked with the BCD396XT, BR330T, BCD996T and a BCT15.