What is the due expiation if somebody is not able to fast on Mondays and Thursdays?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

If a person wants to voluntarily fast on Mondays and Thursdays but cannot do so, then no expiation is due upon him. Rather, if the person misses such fast due to an excuse, then he will still be rewarded for it as long as he made the intention to observe it. The Prophet, sallallaahu ‘alayhi wa sallam, said: “If a slave of Allaah gets sick or travels, then he will still be rewarded for whatever good actions he used to do when he was well and at home.” [Al-Bukhari and others] Of course, Mondays and Thursdays are recommended for fasting.

Allaah Knows best.