Engañar means "to deceive" or "to trick." It's a strong verb that goes beyond simple lying, implying a deliberate act of misleading someone.
In Christian Nodal's song, he repeatedly asks, "¿A quién quieres engañar?" (Who do you want to deceive?). He uses this word to challenge his ex-lover, who is pretending to be over him and happy with someone else, highlighting the self-deception and the futility of her efforts to move on.
"Para Olvidarme" is Christian Nodal calling out an ex who's still pretending she's over him. He opens flat: "Si no encontraste la felicidad no es casualidad" (If you didn't find happiness, it's not a coincidence). Not maybe it's your fault; he's saying it plainly. He knows she's been "Buscarme en alguien más" (looking for me in someone else) and it hasn't worked, because you can't paste someone else's face over a person you're still hung up on.
The chorus turns it into an order: "Ponle una cruz a todos los recuerdos" (Put a cross on all the memories), if she's really trying "Para olvidarme y empezar de cero" (to forget me and start from scratch). He tells her to stop crying over him because "ya está muerto" (it's already dead), stated flat, not cruel. And the song lands twice on the same line: "Para olvidarme no estás lista todavía" (to forget me, you're not ready yet). That's the whole argument. She says she's over him, he isn't buying it, and by the end neither are we.