I’m trying to add funds in Apple Music , but not done

Various forms of this old and somewhat unspecific message ultimately lead you to: https://support.apple.com/billing

The most logical selection is then "Can't complete a purchase?" which brings you to this document --> 'If your payment method is declined in the App Store or iTunes Store'

Click here --> https://support.apple.com/HT203005

If nothing in that document provides resolution then Contact iTunes Support at: https://support.apple.com/choose-country-region/media-services

See the replies to:

purchase could not be completed purchased… - Apple Community

https://discussions.apple.com/thread/8488321?language=en

Your purchase couldn’t be completed - Apple Community