If the Prophet Muhammad (PBUH&HF) is infallible, then why was he told, "May Allah forgive you, why did you…

Answer

In the name of Allah, forgiveness here means overlooking or pardoning something. The Prophet (PBUH&HF), due to his compassion and noble character, permitted some Muslims not to participate in jihad, which was part of his kindness towards people. Allah, by the noble verse, clarified to the Prophet that those he permitted did not deserve such compassion. This did not constitute a sin or error on the Prophet’s part, so forgiveness does not contradict infallibility. Allah knows best.