Here's why:
* "Je" means "I"
* "toujours" means "always"
* "et" means "and"
* "un jour" means "one day"
Putting these together, the phrase would literally translate to "I always and one day." This doesn't make grammatical or logical sense in French.
It's possible the intended phrase was:
* "Je serai toujours là pour toi" (I will always be there for you)
* "Je t'aimerai toujours" (I will always love you)
* "Je suis là pour toujours" (I'm here forever)
It's important to consider the context in which the phrase was used to determine the correct interpretation.