If someone have taken/eaten someone else rights 15-20 years ago but he has realised now that what he did 15-20 years ago was not right so now he wants to apologise or give him back his right that was taken but he can’t reach to that person so what should he do now in this scenario?

Try to find that person if he is not alive then give to children of the dead person.
If still unable to find then give sadaqah on behalf of that person.
If in future he appears, then you must explain to him if he is will be happy then fine. If he demands, then you have to pay.