Seems to me, the verse is talking about, when it already happened, they are unaware. It is the tense of the verse.
The verse does not say, you don't believe it will happen. It says, when it will happen, you are unaware.
I think it's clear the qur'an is talking to the Meccans of the day as it did many hundreds of times. These translations show the warning is being given before the final day, and therefore before it's too late:
Sahih International: And warn them, [O Muhammad], of the Day of Regret, when the matter will be concluded; and [yet], they are in [a state of] heedlessness, and they do not believe.
Pickthall: And warn them of the Day of anguish when the case hath been decided. Now they are in a state of carelessness, and they believe not.
Yusuf Ali: But warn them of the Day of Distress, when the matter will be determined: for (behold,) they are negligent and they do not believe!
Shakir: And warn them of the day of intense regret, when the matter shall have been decided; and they are (now) in negligence and they do not believe.
Muhammad Sarwar: Warn them of the woeful day when the final decree will be issued; they are neglectful and faithless.
Mohsin Khan: And warn them (O Muhammad SAW) of the Day of grief and regrets, when the case has been decided, while (now) they are in a state of carelessness, and they believe not.
Arberry: Warn thou them of the day of anguish, when the matter shall be determined, and they yet heedless and unbelieving.