It sounds like a common complaint.
Check out this website. Lots of info about error 99.
Quantaray is made by Sigma.
Here is a clip from the Sigma website:
Q: Are Sigma lenses compatible with my new Canon Elan 7 camera? The Digital Rebel? How about EOS 10D.
A: All current production Sigma lenses for Canon autofocus cameras are fully compatible with the new Elan 7, Digital Rebel or EOS 10D camera. If you have a question about a particular lens, please contact Sigma's service department (in the U.S.) at (631) 585-1144 or via email at firstname.lastname@example.org <mailto:email@example.com>.
I have the Canon Rebel EOS digital and also the Rebel
35mm. When I purchased the 35mm I also purchased the Quantaray 70-300 zoom lens. This purchase was over 5 years ago. I have tried using the lens on my Digital but keep getting an "Error 99". Is the lens too old for this camera? I have no photo stores close by to have it checked and I'm wondering if it is just not compatible.