All humans have sins, so how do we say in the funeral prayer, "O Allah, we know nothing of him except good"?

Answer

In the name of Allah, know, my son, that every deceased over whom we pray, we know that he died a Muslim and a believer in the guardianship of the Ahlulbait (PBUT). This is the good we are certain of. As for the sins he had, we assume that he repented from them, and we do not know of his lack of repentance. For whoever is in this condition, we know good of him and do not know evil of him. Allah knows best.