[The Reason for Supplicating with His Saying: “Our Lord, Do Not Take Us to Account If We Forget…” — the Verse]

Answer: It is rationally possible to take to account for error and forgetfulness; and if it is possible, then supplication to Allah for not taking to account for it is directed.
The clarification of that: the legally responsible person can avoid falling into error and forgetfulness by avoiding the causes that lead to falling into that. For that reason Allah—Exalted is He—punished Adam for his slip, and it had occurred from him in the manner of forgetfulness, as indicated by His—Exalted is He—saying: "And he forgot; and We found not in him determination" [Ta-Ha:115]. And Allah—Exalted is He— reproved our Prophet Muhammad (May Allah bless him and his family and grant them peace) for his granting permission to those who stayed behind, so He—Glorified is He—said: "May Allah pardon you; why did you give them permission…" [At-Tawbah:43].
And His—Exalted is He—saying: "And do not burden us with that which we have no ability to bear" can be said to be a metonymy for difficult obligations; meaning: do not burden us with difficult obligations.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3