My Apple Store won’t let me do in game purchases

How in-app purchases work: Buy additional app features with in-app purchases and subscriptions - Apple Support

You may be restricted in what you can do through parental-controls, or through Family Sharing, or ScreenTime.

If so, you may want to discuss the restrictions with the Family organiser, ie: to ensure money is spent wisely.

Else: You can check if there are restrictions set on in-app purchases:

Manage App Store purchases, settings, and restrictions on iPad - Apple Support

or

If you : Use Screen Time to prevent purchasing - Apple Support

What to do: If your payment method is declined in the App Store or iTunes Store - Apple Support

or

Could there be a problem with a previous purchase?:

See: If a message says 'billing problem with previous purchase' or 'verification required' - Apple Support

All the best :-)