Cough syrup. I can't relax unless I take a sip before a performance!
Literal
Cough-stop syrup. Real-performance before [to-に] one-sip drink-not-[conditional-と] can't-settle-down!
The fragment 咳止めシロップ opens with 体言止め — a bare noun declaration. 本番 ('the real thing, actual performance') contrasts with practice/rehearsal. 一口 ('one sip') is the counter for mouthfuls. ~ないと落ち着かない ('can't relax unless I do X') expresses compulsive dependence. The humor is in the ritual — needing cough syrup as a pre-performance ritual.