His portal has been destroyed like every others, has it has been stated in season 5, But Jack will still be able to get bac in time using Aku's own Magic, but alone, without Ashi. And win.
But years in his own future (so way back in feudal Japan), After he has become King of several continents after having freed the world of Aku's domination, he will long for Asho and realize that most of his life has been in the future. So, after his parents died of old age, he decide to go back to the future and it is ONLY then that he finally get to use the Guardian's time portal, to get back to the future and rejoin Ashi.
This is why he couldn't use it yet beforehand: Because he had already used it in the past and JAck hadn't get to the point yet where he was going to use it in the past.
Some time-travel fuckery.