Am I sinful, or is charity obligatory upon me, for a car accident in which I caused a scratch to a wrongly parked car, and its owner left without my knowing whether he noticed the damage or not, knowing that my car was damaged more?

You are not sinful, for the error is forgiven. As for guaranteeing damage, the default is that you are liable unless there was an encroachment in parking by the other party, and the traffic department determines this. If you are liable and cannot find the car owner, then estimate the value of the damage and give it away as charity on their behalf.