What’s the meaning of the word Yaqin “certainty in the verse “And worship your Lord until there comes to you the certainty”? Does it mean death? Why can’t it be interpreted as the prophet’s ascending to Heaven during the Isra’ Journey as he saw the Great Signs of his Lord?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.
It is known in Arabic that the word “until” is used to indicate the reach to an end. Thus, the verse urges Prophet Muhammad to keep worshipping his Lord until the last day of his life. Therefore, the word Yaqin or certainty is interpreted as death. In addition, if it is interpreted as the journey of Isra’, it means, then, that Allah wants His prophet to worship Him until that night, so what shall the prophet do after that? Didn’t he need to do any act of worship? The prophet as well as humans need to worship their Lord until the end of their lives. 
To conclude, what the interpreters who explained the Holy Qur'an said is accurate. Worshipping Allah till the last day of one’s life is the tradition of prophets as well as their followers.