Scholars of Tafsir have mentioned three main meanings here:
1- Sin means Tark Awla. The Prophet (s) would ask Allah to forgive him for not doing more good deeds, though everything he did was good. Sometimes you complete a task beautifully, but then you go to your superior and say, “Forgive me for not doing it better.”
2- Sin refers to the sins of his people, and since he is their leader then he feels responsible for them, and so he asks for forgiveness on their behalf.
3- Sin means the sins that his enemies think is a sin. By preaching Islam, the pagans attacked him and fought. They considered him sinful by not worshipping their idols. So Allah tells him don’t worry I’ll forgive you this “sin” which means I’ll protect you from you enemies.