The Lost Ark in-game store use the Steam Overlay to complete your purchase. If the overlay is disabled, you’ll get a momentary translucent box behind the dialogue where you click “Buy Now”, and it’ll close in a few seconds. That’s an indication of it trying to load Steam.
To check, open the Steam client, click File and select Settings.
Chose In-game and ensure “Steam Overlay” is checked.
Please note, you’ll have to restart Lost Ark for it to take effect. In other words, the Steam Overlay won’t be enabled until you relaunch the game.
The problem for myself and many others is that the Steam Overlay is ON but it isn’t working AT ALL in Lost Ark and only in Lost Ark.
I tested it out with other games and it works just fine. I even opened up Valheim at the same time (so both LA and Valheim are running) and in Valheim it works with no issues. In Lost Ark it just doesn’t want to show the overlay.
UPDATE:
Well well well…i guess it’s a localization issue.
I ran the game with a VPN pointing to Romania…and the overlay works just fine. Can now buy stuff.