The Provider needs to set up dependency of domain subscriptions on hosting subscription.
E.g. the Provider sells the domain registration service and they want that customers host purchased domains on the Provider's servers only. So, the Provider needs to configure PBA to sell domains to customers only if they have at least one subscription with hosting services (shared web hosting, mail hosting, application hosting, VPS hosting, etc.).
The Provider already configured domain service plans as up-sales to hosting service plans, so hosting subscription automatically becomes parent to the domain subscription, however it is not enough. Still, a customer may be able to subscribe only to a domain service plan without purchasing hosting services from the Provider.
Enable the 'Parent Subscription required' property in the domain service template(s) in PBA. After that customer will not be able to subscribe to the domain service plan based on the service template(s) configured such way until parent hosting plan is purchased and provisioned.
Refer to the PBA Provider Guide for more details - Preparing Domain Service Template.