Aguanta comes from the verb aguantar, which means to endure, tolerate, or hold on. It's a versatile word that often implies a struggle or a limit being reached.
In this song, the lyric "esa relación ya no aguanta más" (that relationship can't take it anymore) uses aguanta to convey that the relationship has reached its breaking point and can no longer sustain itself. It's a powerful way to express the end of a romantic connection.
This song is all about dealing with a breakup, but with a twist! Christian Daniel sings about his partner saying, "dices que te vas, que esa relación ya no aguanta más" [you say you're leaving, that this relationship can't take it anymore].
But instead of being heartbroken, he's like, "y si tú te vas, yo no voy a llorar, yo no voy a sufrir" [and if you leave, I'm not going to cry, I'm not going to suffer]. He even says, "si tú te vamos a celebrar en un ratito, te voy a olvidar" [if you leave, we're going to celebrate in a little while, I'm going to forget you]. He plans to "beber toda la noche" [drink all night] and go out dancing. It's a pretty defiant take on moving on!