To do Alt + A U on Mac in Excel, download and install Accelerator Keys, then press Option, followed by A, U.
Command: Ungroup Menu
This is how to do Alt + A 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 opens the Ungroup dropdown menu in Excel’s Outline section. This menu provides options to ungroup selected rows or columns, or to completely clear the outline structure from your worksheet.