بسم الله الرحمن الرحيم
It is not necessary and wajib to offer tahajjud salah rather it is a nafl prayer which reports lots of virtues in hadith. Thus if someone offers tahajjud then it is very virtuous. But if tahajjud is missed due to illness or journey then there is no sin.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.