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If the lens contacts on the camera mount of the used EOS 7D are not making good contact with the Canon lens's corresponding contacts, autofocus adjustments won't be made through those contacts.
Clean the contacts, on both lens mount of camera body, and lens mount of lens, with a soft dry cloth, wiping gently to avoid rubbing off the sensitive coating on the contacts.
Then try again.
If it still fails... either the lens or the camera body has a problem. How to tell which one? try another camera body with that lens, or try another lens with that camera body.
Also: is the 'Canon lens' truly compatible with the EOS 7D body? tell me its model and I will look up compatibility for it and the EOS 7D body.