Is Jupiter a failed sun?
No, Jupiter isn't massive enough to be a star.
There are two main ingredients to make a star. It needs to be mainly hydrogen and it needs to be massive enough for gravity to compress the hydrogen enough for fusion to take place.
Jupiter is mainly hydrogen but it is only about 1000th of the mass of the Sun. Although there are stars smaller than the Sun, Jupiter would need to be about 100 times more massive for fusion reactions to take place in its core.