The statement: فَلَنَسْأَلَنَّ الَّذِينَ أُرْسِلَ إِلَيْهِمْ وَلَنَسْأَلَنَّ الْمُرْسَلِينَ ﴿6﴾ in verse 6 refers to the day of Qiyamah when people will be asked by Allah how did they receive the messengers and Books sent to them by Him - and the messengers will be asked if they did convey to their communities the message and injunctions Allah had sent them with. (Narrated by Baihaqi from Sayyidna Ibn ` Abbas ؓ - Mazhari)
It appears in Sahib Muslim on the authority of a narration from Sayyidna Jabir ؓ that the Holy Prophet ﷺ asked people during the address of his Last Hajj: When, on the day of Qiyamah, you will be asked about me - whether or not I have conveyed the message of Allah to you - what will you, then, say in reply? (فَمَا انتم قَائِلُونَ ). All Sahabah present there said: We will say that you have conveyed the message of Allah to us and you have fulfilled the trust of Allah at its best and you have wished and done what was most beneficial for the community. Hearing this, the Holy Prophet ﷺ said: اَشھَدُاَللَّھُمَّ Allah, You be witness).
According to a narration in the Musnad of Ahmad, the Holy Prophet ﷺ is reported to have said: On the day of Qiyamah, Allah Ta'ala will ask me if I have conveyed His message to His servants and I shall say in response: Yes, I have. Therefore, all of you should make sure that those who are present do convey my message to those absent. (Mazhari)
` Those absent' refers to people who were present during that time but were not present at that particular occasion - as well as generations which would come later on. To convey to them the message of the Holy Prophet ﷺ means that people of every time should continue the chain of conveying this message to the generations coming next, so that all children of Adam to be born right through the day of Qiyamah will have received this message.