Why can't I access Apple Music with my child's iCloud account?

Set up account for my child, sharing apple one subscription, can log in to the iCloud and messages with my child's account but when I try app store or itunes it shows 'purchase could be completed'.

Formatting iphone = no effect. Logging in with parent account (works) then trying with kids account (doesn't) = no effect. App store payment methods and billing are all correct and in order. All subscriptions shared and family group set up...

Any ideas?

[Re-Titled by Moderator]