Translation:It is difficult to get a promotion.
It is difficult to get promotion/It is difficult to get a promotion.
In UK English the first translation (which I would regard as the correct one here) is a generalisation in the sense that no promotion is ever easy. The second one refers to a particular promotion, or a particular employer, implying that other promotions are easy but this one is difficult.
For example, you might say "It is easy to get promotion in the administrative grades but a promotion to managerial grade is difficult"