Why does it say error can not complete transactions when trying to purchase stuff in games

See -> If you can't make purchases or update apps, including free apps, you need to update your payment method or add a new one. 

See also ⤵️

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

Check also ⤵️

1.

If you see a message that says "Your account has been disabled in the App Store and iTunes", contact Apple Support for help.

2.

If your payment method is declined in the App Store or iTunes Store

If your payment method is declined in the App Store or iTunes Store - Apple Support

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Get more help

• If you disputed a charge from the App Store or iTunes Store, contact your financial institution again to authorize that charge and future purchases from the App Store or iTunes Store.
• If you're outside of the United States, you might need to contact your financial institution to allow international purchases.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• If you still need help, contact Apple Support.

3.

If your Apple ID is locked or disabled

If your Apple ID is locked or disabled - Apple Support (CA)

-> Get a verification code and sign in with two-factor authentication

Get a verification code and sign in with two-factor authentication - Apple Support

4.

If you can't connect to the App Store, iTunes Store, or other Apple services

If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

-> If you tried all of the above steps and you still can’t connect, contact Apple Support.