What is error 99

Error 99 is a very general error meaning it could be several problems try this to track down exactly what the problem is .
1. Try your other lens and if it takes pictures just fine and auto focus is ok also its a problem with your Kit 18=55 mm lens.
2. Try cleaning the contacts on your kit lens using a pencil rubber or a cotton bud with a tiny amount of lens cleaner on it be careful not to let any of the rubbers dust into your lens.Replace the lens and see if the error 99 still comes up if it does then try this.
Take out both batteries the main and the lithium 2052 for at least 20mins.Replace and then try taking out the memory card if it still takes pics then your card is ok replace the card and try using the lens in manual mode if that takes decent shots then its the lens auto focus system.
As long as the mirrors look ok and it works with your Sigma lens its a fault with the kit lens.Try going to your local camera shop and fitting a kit lens that works and it should stop the error 99 if none of the advice works your camera needs a service.
Hope this helps i had the same prob and it was the kit lens and not the mirror system its usually one or the other with that fault number.

What wide angle lens is it need f number it may be to old to work on eos 400d

Error1