Is one who witnesses an evil act but is unable to admonish its perpetrator sinful, and what is the expiation for that?

Enjoining what is right and forbidding what is wrong is a religious obligation and a distinguishing characteristic of goodness for Muslims. If you are unable to reach the person you intend to advise, then there is no sin upon you, because obligations are tied to ability, and there is no burden beyond what one can bear. Moreover, a good intention is hoped to bring reward, even if a person is prevented from doing good due to an excuse.