Why can't Sett - Brawler conquer Monastery of Hirana, get the buff from conquering, and then spend that buff to draw?

You cannot conquer the Monastery of Hirana with Sett, Brawler and use the buff he gains from that conquest to pay for the Monastery's draw effect.

The Correct Sequence

When Sett, Brawler conquers the Monastery of Hirana, both abilities trigger simultaneously. According to Rule 383.3.b.1, the cost to spend a buff must be paid in order to finalize the Monastery's triggered ability to the chain.

1. Trigger Placement: Because the cost must be paid to place the Monastery's trigger on the chain, you must already possess a buff at the moment of conquest.
2. Cost Payment: You spend an existing buff to place the Monastery's draw trigger on the chain.
3. Chain Resolution: Sett’s trigger is then placed on the chain. When it resolves, he gains a new buff.

Why this is required

• Cost Timing: Per Rule 383.3.b.2, costs for triggered abilities are not paid upon resolution; they are paid when the ability is placed on the chain.
• Pre-existing Buffs: You must have a buff available before the conquer event occurs to satisfy the cost requirement for the Monastery's ability. You cannot use the buff gained from Sett's trigger to pay for the Monastery's trigger, as the Monastery's trigger must be finalized on the chain before Sett's trigger can resolve.

Summary: You must have a pre-existing buff to trigger the Monastery of Hirana's draw effect. You cannot use the buff gained from Sett, Brawler's conquer trigger to pay for the Monastery's cost, as the cost must be paid when the trigger is placed on the chain, prior to the resolution of Sett's ability.

Note: I could not fully verify this ruling against the retrieved rules.