a. Check to see if the camera's lens is mounted correctly in step one.
b. Using a microfiber cloth, a soft-bristled brush, or an air blower, clean the contacts between the camera and lens. Get rid of any debris on the lens, including dust, oil, fingerprints, and smudges. For cleaning, you can use utilise fluid for DSLR lenses.

Caution:

Avoid cleaning the lens with the microfiber cloth straight away. With an air blower and brush, first remove any dirt.
Avoid using a moist cloth to wipe the contacts.
Never clean the lenses with cotton swabs or any kind of liquid fabric softener.

Source : https://www.repair-service-center.com/canon/canon-service-center.php