Apple one subscription

1. Which Apple One Subscription did you get? You need Family or Premium to be able to share Apple Music.
2. Check on their devices that the subscription is visible in Settings->Family Sharing.
3. Did they have their own subscription previously and cancelled it?

If they had a subscription and cancelled it recently, they are not eligible for the free trial on Apple One.

Access will be available once the free trial is over and you are actually charged.

See here: link-> Bundle Apple subscriptions with Apple One - Apple Support

Apple One includes a free 1-month trial for services that you don’t already have. After your free trial, services that you previously subscribed to are automatically included in Apple One. If you recently canceled a service, you might not be eligible for the free trial. Your plan automatically renews after the trial.

Did you actually physically cancel it? Or did you just setup Apple One directly? If you just set it up, then it would have transitioned automatically with no actual cancelation in between.

If you did manually cancel it before getting Apple One, then that would be a mystery. Maybe dependent on how much time passed between the cancellation of one and the activation of the other.