There are certain in-game features, such as the ones used for purchasing VP, which will only work if certain Steam client settings are enabled. Make sure to check if the 'Steam Overlay' is enabled on your Steam client. To do this, follow these instructions:
1. Located at the top part of your Steam client, click on 'Steam'.
2. Under 'Steam', click on 'Settings' to bring up a new window.
3. On the Settings window, click on 'In-Game'.
4. Under 'In-Game', make sure that a checkmark is seen on the 'Enable the Steam Overlay while in-game' option.
Once Steam Overlay is enabled, please relaunch the GrandChase Classic game client and check. A new purchase window from Steam will appear to allow you to continue with your purchase.