Allah says, “If they, when they had been unjust to themselves, had come to you (Muhammad (ﷺ)) and begged Allah's Forgiveness, and the Messenger had begged forgiveness for them: indeed, they would have found Allah All-Forgiving (One Who accepts repentance), Most Merciful.” [1]

This ayah is often used to support the stance that in addition to seeking forgiveness from Allah by ourselves, we also have to seek forgiveness through the Prophet (ﷺ) i.e, intercession through the Prophet (ﷺ).

Could you kindly clarify?

"......when they commit errors and mistakes, come to the Messenger, so that they ask Allah for forgiveness in his presence and ask him to supplicate to Allah to forgive them. If they do this, Allah will forgive them and award them His mercy and pardon. This is why Allah said,(they would have found Allah All-Forgiving (One Who forgives and accepts repentance), Most Merciful)."[2]

It's clear that this was during the lifetime of the Prophet (ﷺ).

References:

[1] [Surah An Nisa’, 4:64]

[2] [Ibn Katheer]