The basic steps to resolve the issue require you to activate your Steam Overlay. Don't worry, this is a quick and simple process and all done through the Steam client in a just a few minutes.
In order to get everything working, please follow the below steps:
1. On the Steam client, go to Steam -> Settings
2. Click the In-Game tab
3. Make sure "Enable Steam Overlay while in-game" is checked
4. Go to your Library
5. Right-click on Black Desert Online
6. Under the General tab also make sure "Enable Steam Overlay while in-game" is checked
After that, you will be able to claim your Game Package goodies by opening the in-game ESC menu and pressing the "Redeem" button.
The goodies will be sent to your in-game mail (press B). Please note that some items will be sent to your Pearl inventory tab instead of the normal tab.