What is the sin incurred by serving alcohol at an external event due to work pressure and management's request, despite knowing that there are designated people for that, and feeling remorse for the action?

You erred by going to the place where alcohol is served and by helping to serve it to people. This is forbidden, for the Prophet, peace and blessings be upon him, said: "Allah has cursed alcohol, its drinker, its server, its seller, its buyer, its presser, the one for whom it is pressed, its carrier, and the one to whom it is carried." Therefore, you must repent sincerely and seek another job where you are safe from temptation, because whoever seeks the pleasure of people through the displeasure of Allah, Allah entrusts him to people.