Canon Rebel EOS camera

Err 99 is a general error msg. That is, it has to be connected to the service soft that only Canon self or an ASF has, to know more precisely the problem.
To narrow the possibilities;
-choose Av, shutter priority,
-choose the smallest aperture your lens allows, 22 or 29 or so,
-while pressing and releasing the depth-of-field preview button, (front, under the lens release) look into the lens from the front.
If the diaphragm doesn't move it has to be changed. If you have the 17-55 standard lens; throw it away and buy a better one instead of paying a repair. You'd be better off.

If you have the Err msg with several different lenses the problem is in the body itself, often the shutter. Check also your PINs in the CF-slot. If one PIN is toching another; break off the bent one and normally you can go on, the PINs are almost all doubled.