iTunes music purchase

A question: do you use the same Apple ID on your iPad & iPhone?

If you get a message -> Your purchase could not be completed, you need to update your payment method or add a new one. 

See the following Apple articles:

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 Account balance – Apple Support (UK)

For more support and assistance click here

I will try and add another form of payment.

Keep in mind: if you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

Thanks for your feedback & you're welcome.