Can you cast a sorcery during your first main phase before your opponent's upkeep trigger resolves?

This question rests on a fundamental misunderstanding of turn structure. Each player has their own full turn, complete with their own Beginning Phase (Untap, Upkeep, Draw), their own Main Phases, Combat Phase, and Ending Phase. Your first main phase and your opponent's upkeep exist on entirely different turns — they never overlap (CR 500.1, 501.1, 505.1).

During YOUR turn, your first main phase comes after YOUR beginning phase (YOUR untap, YOUR upkeep, YOUR draw step). At this point, your opponent's upkeep is not happening — it either already happened on their previous turn or will happen on their next turn. There is no scenario in a normal game where both occur simultaneously (CR 505.4).

You can cast a sorcery during your first main phase if the stack is empty and you have priority (CR 307.1). However, this has nothing to do with your opponent's upkeep trigger, which simply isn't active at that time.

If you meant to ask whether you can cast a sorcery in response to a trigger that occurs during YOUR upkeep — the answer is also no, because sorceries can only be cast during a main phase when the stack is empty (CR 307.1).

Example: Your opponent has a Phyrexian Arena that triggers at the start of their upkeep on their turn. You cannot cast a Divination during their upkeep to 'race' that trigger — it's their turn, and you can only cast sorceries on your own main phase with an empty stack.