Hi WesVn,
Thank you for using the Apple Support Communities! I understand you are unable to make in-app purchases within a specific game. I'd like to help.
The first step I would recommend, is to check if you have any restrictions set up to prevent in-app purchase: Prevent in-app purchases from the App Store - Apple Support.
If everything is clear with your restrictions, and you are able to make other in-app purchases but it fails for this specific game only, I would recommend trying these steps: If an app unexpectedly quits, stops responding, or won’t open - Apple Support. The last step being to reach out to the app developer to report the issue: How to contact an app developer - Apple Support.
If you are unable to make any in-app purchases in multiple applications, I would suggest checking and confirming that your payment details are accurate and up to date, as suggested in this article of support: If your payment method is declined in the App Store or iTunes Store - Apple Support.
Best regards.