Why do you get error 99 on a canon 20D?

Usually, card or lens problem. Remove the CF card, put it back, if it doesn't work, verify card on other camera or card reader. If the card is ok, then the lens may not have good contact, or faulty. Remove lens from body, clean electrical contacts from lens mount and also from body. It may happend more often when you have a different lens maker (Sigma, Tamron..).