that's just it

that's just it

That's precisely the issue. A: "You have to be sponsored for a work permit by a company in the country where you wish to live." B: "But that's just it—I can't get a job with any companies there unless I have a work permit to begin with!"
See also: just

that’s just ˈit

(also that’s just the ˈtrouble) (informal) that is exactly the problem: ‘You only need to spend £5 more and you can get a really good dictionary.’ ‘That’s just it, I’m afraid — I haven’t got another £5.’
See also: just