Can you cast a sorcery during your second main phase after combat?

Sorceries can be cast any time you have priority during a main phase, provided the stack is empty and it is your turn. This is defined in CR 307.1, which states that a player may cast a sorcery only during a main phase of their own turn when the stack is empty.

A typical turn has two main phases: the first main phase (before combat) and the second main phase (after combat). Both satisfy the conditions of CR 307.1 equally. There is nothing in the rules that treats the second main phase differently from the first for the purpose of casting sorceries.

The second main phase (CR 505) occurs after the end of combat step and before the end step. Once combat concludes, you enter your second main phase with full permission to cast sorceries, play a land if you haven't already, and take any other main-phase actions.

Concrete example: You attack with a creature during your combat phase. After combat resolves and the end of combat step ends, you enter your second main phase. You may now cast Cultivate (a sorcery) — the phase and stack conditions are met, so this is perfectly legal.

This is a common strategy: holding a sorcery until after combat lets you see the outcome of the combat step (e.g., whether your creature survived) before spending your mana or resources on it.