Unable to complete purchase

The message is clear.

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

See also ⤵️

Add money to your Apple ID balance

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

Payment methods you can use with your Apple ID

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

See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

If you have add funds to your Apple ID balance, it would take about 1 to 3 business days.

Then check in a few days again