Hi James. Welcome to justanswer.
Try changing the lens and then see if the error is still there or not.
Hi. My name is*****'m a camera repair technician with almost 30 years in the busines and I'll be glad to help you with your question today.
Error code "99" is a general code that simply means there is something wrong with the camera and it should be serviced by a qualified technician. It's usually a bad shutter which can be replaced. Canon no longer services the 50D as it has been out of production for several years but parts are still available from many sources and there are many qualified, independent shops that still work on them. . 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.