User is logged in with an Owner account of a Core company and has full security access, but cannot do billing from the Invoices screen. The screen displays a message that the user is not subscribed to the module.
This can happen if you do not have the subscription (say Billing or Core Essentials) that allows you to use the Invoices screen. Even though you have full security access, you must also have the appropriate subscriptions assigned to access a module. Security permissions only work within the subscriptions assigned to you. Check out this article to know more: Subscription, Security and Employee Control.