In Domains > example.com > Databases the button Add Database is missing.
The maximum number of allowed databases for service plan is exceeded by subscription.
Open settings of service plan for subscription example.com in Service Plans > example_service_plan > Resources tab and increase number in Databases field.
Alternatively, it is possible to customize this setting only for one subscription by navigating Subscriptions > example.com > Customize > Resources and increasing number in Databases field. In this case, this subscription will become in a Locked state and will not synchronize with changes of its service plan.