The Capacity settings tab allows you to manage the Power BI Premium and Power BI Embedded capacities of your tenant. Users will only be able to see capacities if they have been assigned permissions, such as the appropriate capacity administrator role:
Power BI Premium instances can be purchased from the Microsoft 365 Admin Center by clicking on the Purchase button in the Power BI admin portal | Capacity settings. Power BI Embedded capacity, however, must be purchased from the Azure portal.