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.