Pecao' is a colloquial, shortened form of "pecado," which means "sin" or "shame."
In this song, Manuel Turizo uses it to express that it's a "sin" or a "shame" for the woman to be alone, especially when he believes he could make her happy. It's an engaging word because it's used in a playful, slightly provocative way to emphasize his desire and the perceived injustice of her being by herself.
"Qué Pecao" is Manuel Turizo and Kapo talking a woman out of staying guarded after a bad relationship. He names the reason up front — "Por un cabrón cree que todos somos iguales" ("Because of one asshole you think we're all the same") — and spends the song arguing against that math rather than pushing past it. His opening ask is modest: "Dame una noche, aunque sea pa' que hablemos" ("Give me one night, even if it's just so we talk").
The hook turns the title into the argument itself: "Que duermas otra noche sola es un pecao'" ("That you sleep another night alone is a sin"), casting her staying single as the actual loss, not a safe choice. By the second half he raises the stakes — "Si la vida me cumple un deseo" ("If life granted me one wish"), "Pediría que tú me quisieras" ("I would ask that you love me") — before Kapo signs off with his own name: "Yo soy tu Kapo, baby" ("I am your Kapo, baby").