Please contact iTunes support to complete this transaction

Try to verify your Apple ID details:

Sign in to your Apple ID account page and edit / update your Apple ID details.

https://appleid.apple.com

You need to have a payment method on file, even if the app you want to download is free, ->

Check which payment method you have on file 

• If you want to change your card, add first the new card, then remove the old one.
• or if you use your Apple ID balance, set your Apple ID balance as your default payment.

See also -> Payment methods you can use with your Apple ID

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

You can also use your Apple ID balance as payment method: Add money to your Apple ID balance

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

Click here for more support and assistance.