Fasting the six days of Shawwal is Sunnah and not Wajib whose reward is hoped for even if one fails to complete due to a lawful excuse
Q: I started fasting the six days of Shawwal when some circumstances and business interrupted me. I could not fast the remaining two days. What can I do? Should I make up for missing these two days? Will I incur a sin for not completing the six days?
A: Fasting the six days of Shawwal is a Mustahab (desirable) act of worship but is not Wajib (obligatory). You will receive the reward of the days on which you fasted and hopefully you will receive the full reward of fasting the six days, if you had a Shar‘y (Islamic legal) excuse for not completing them. The Prophet (peace be upon him) said: When a servant (of Allah) falls ill or travels, the same reward of the good deeds they used to do when at home in good health is recorded for them. (Related by Al-Bukhari in his Sahih [Authentic Hadith Book])You do not have to make up for those two missed days. May Allah guide us all to success!