( فَلَمَّا كَشَفْنَا عَنْهُمُ الرِّجْزَ إِلَىٰ أَجَلٍ هُم بَالِغُوهُ ) (But when We removed the punishment from them until a term which they were to reach): This is the preordained decree of Allah (swt). Allah (swt), with His (swt) Foreknowledge, knew that they would break their covenants with Prophet Moses (PBUH). Thus, Allah (swt) relieved them from these punishments (until a term), meaning it was postponed until a specific period of time.

 (which they were to reach): They received the punishment when their time came, when Allah (swt) drowned them in the sea.