To do Alt + A U U on Mac in Excel, download and install Accelerator Keys, then press Option, followed by A, U, U.
Command: Ungroup
This is how to do Alt + A U U on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + A U U ungroup removes the selected rows or columns from their outline group. This expands the outline level without removing other groupings in the worksheet.