Hi. My name is*****'m a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
While the 1D 2n is an excellent camera, it is an older model with probably more than a few shots on it.
Error code "99" js a general purpose code that simply means something is wrong with the camera and it needs serviced. The internal mechanisms are very intricate and subject to wear and tear. Among these is the shutter unit and the charge motor assembly. If either of these mechanisms are worn and out of tolerence the camera will produce the error you are experiencing. This really isn’t a user serviceable problem and should be repaired by a qualified technician. You may want to check locally for a camera repair shop in your area, or, if you need assistance finding a shop, please let me know and I'll locate one for you.