Sinsabores literally translates to "without flavors," but it's used to mean disappointments, bitter experiences, or sorrows. It's a more poetic and less common way to express hardship than typical words like problemas.
In the song, the singer reflects on wanting to relive his life, but without the "tantos sinsabores" (so many bitter experiences) of the past 20 years. This word adds a layer of depth and regret to his longing for a different past.
Have you ever wished you could turn back time and relive a moment, knowing what you know now? That's exactly what Agua Marina sings about in "20 Años Menos"! The singer dreams of going back twenty years to meet his partner all over again. He's so sure of their love that he says, "quisiera que mi vida regresar hacia el pasado tener 20 años menos y volverte a conocer" [I wish my life would go back to the past, to be twenty years younger and meet you again]. He'd immediately ask her to be his wife, certain he wouldn't change a thing.
He wants to experience another twenty years with her, even with all the "sinsabores" [bitterness] and "días de placer" [days of pleasure]. He imagines them growing old together, calling each other "tiernamente" [tenderly] until their old age. The song really emphasizes this idea, repeating how he wouldn't hesitate if he had the chance to choose her again, even if he had the "fortuna de todas las mujeres" [fortune of all women]. He'd pick her every single time, making her his girlfriend and then his wife, without a moment's doubt!