There are coupons you will get from events, achievements, where they can’t be used.
Now we will talk about how to use premium outfit coupons.
At August of 2019, pubg lite was newly released, having outfit events. Where you can use outfit coupons as redeming them and getting random items.
But as of now, you cannot use those coupons, because the event has closed.
So most probably, to use those premium coupons, one must wait for aniversary of pubg mobile lite game.
Stack em up, and blast them when event arrives!