If what the Prophet (PBUH&HF) did was by divine command or revelation, then why did Allah say "Allah forgive…

Answer

In the name of Allah, be advised, my son, there are many instances where Allah (The Exalted) allowed the Prophet (PBUH&HF)not to prohibit certain things out of His compassion and mercy towards him. However, the Prophet (PBUH&HF) was compassionate and merciful towards all humanity and even beyond. He bore pains and various forms of suffering in this cause. Perhaps Allah, The Highest, allowed him not to do or endure this severe burden as an honor and mercy towards him. However, he did so. In such instances, this expression was used, meaning 'May Allah not hold you accountable for it.' Allah is the Guide, and the Knower.