You will need to log into your game account on the Web Store site even if you are currently logged in to the app. You may either log in by tapping “sign in” on the top right corner of the Web Store page or you will be prompted to log in when checking out with your items.

After logging in, you'll find your current in-game currency balance at the top right of your screen. You can also access your Player Profile information and payment history for the Web Store you're signed into.