Faltes comes from the verb faltar, which has several meanings including 'to lack', 'to miss', or 'to be absent'. In this song, it's used in a plea, meaning 'don't fail me' or 'don't be absent from my life'.
Rodrigo Tapari uses this word repeatedly in the chorus,
Get ready to feel the love with Rodrigo Tapari's "Nunca Me Faltes"! This song is a heartfelt declaration of deep affection and a plea for unwavering presence.
He wants to whisper "cosas preciosas" [precious things] to his beloved, telling her how much he loves and adores her, and that she's the woman he's dreamed of. She makes him incredibly happy, and when she's not around, he feels like he "me falta el aire" [can't breathe]. For him, she's the very reason he exists. The chorus is a direct and passionate request: "Amor por favor, nunca me faltes" [My love, please, never leave me], because "tu eres todo para mí" [you are everything to me]. He also begs her, "nunca me engañes" [never deceive me], emphasizing that "sin tu amor yo no soy nadie" [without your love, I am nobody]. It's a powerful and direct expression of how completely his happiness and identity are tied to her.