Canon A520 have CR1220 battery as memory backup power. Replace it. It's located at left hand side there is grey rubber cap open it, you will find one more small flap at bottom, gently pull out straight you will see a battery holder remove CR1220 battery replace same type with new one. Normally it lasts for 3 years.
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Try downloading the current software for you model from the Canon site. You could try inserting the memory card into the card reader on your computer and if you get the same result then it would appear to be a software problem that you use to download. Sometime conflicts appear.
work for about 50% of lens errors, but they're worth a try. The list includes:
Remove the batteries and wait a few minutes
Try pressing and holding the Menu, Function, Function Set, or OK while turning on the camera, or otherwise find a "factory reset" option.
Try turning the camera on without a memory card inserted
Place the camera flat on its back and turn it on while holding down the autofocus button, to try to cause the camera to attempt autofocus while the lens is extending
Blow compressed air in any gaps around the lens - or use a shop vac
Use a thin piece of paper or a needle to dislodge any visible bits of grit
Gently tap the camera in an attempt to realign jammed mechanics or dislodge particles (obvious warning: could cause further damage)
Gently push or pull the lens barrel, as a last resort (again, potential for further damage).