Translation:

21. And indeed We will make them taste of the Penalty of this (life) prior to the supreme Penalty in order that they may (repent and) return.

Notes (Tafseer)

3654. The final Penalty is to come in the Hereafter. There is no doubt about it. But before it comes, a minor Penalty comes in this very life. It may be in some kind of misfortune, or it may be in the pangs of a tortured conscience or secret sorrow. But this minor Penalty may be really a mercy, as it gives them a chance of repentance and amendment.