Hi MattyIce5643,
Thanks for reaching out here in the Apple Support Communities. I understand you are getting an error message while trying to make an in-app purchase. I'm glad to help with this.
You've done good troubleshooting by contacting the app developer about this. Before reaching out to Apple Support, there are some troubleshooting steps that you can work through first, which may help resolve the issue. Be sure to test out making the in-app purchase after each step:
1 - Close the app following these steps: How to force an app to close on your iPhone, iPad, or iPod touch. Launch the app and try the in-app purchase.
2 - Restart your iPhone.
3 - Check for updates to the app and your iPhone.
4 - If the issue persists, reach out directly to Apple Support about this, so they can take a closer look: Apple - Get Support
Best Regards.