If you miss Ashura, you cannot make up for it later, because the virtue of fasting Ashura is connected to a specific day. It is intended to fast during that day because of its special occasion and events.

 

But there is a hadith of the Prophet which confirms that “When a slave of Allah suffers from illness or sets on a journey, he is credited with the equal of whatever good works he used to do when he was healthy or at home”. (Al-Bukhari)