Thanks for reaching here! Could you please confirm your billing account type.
As an invoice is generated based on your billing account type. Invoices are created for Microsoft Online Service Program (MOSP) also called pay-as-you-go, Microsoft Customer Agreement, and Microsoft Partner Agreement billing accounts. Invoices are also generated for Enterprise Agreement billing accounts and In some cases invoice generated is on monthly basis. Let us know to help you better on this.
To identify your billing account type, see View billing accounts in Azure portal.