Where in the Matomo UI can I create a Roll-Up measurable?

You can create Roll-Ups in the Websites Management the same way you would create a regular website or app. Once installed, the menu item in the Administration might be renamed from “Websites” to “Measurables”. To create a Roll-Up go to “Administration => Measurables: Manage”. Next click on “Add a new measurable” and in the dialog choose “Roll-Up”. Then you can define the name and choose which of your websites and apps should be grouped together under this Roll-Up. For more details check out our Roll-Up Reporting Guide. Please note that Super User access is needed in order to create a Roll-Up measurable.