Excel ribbon shortcut in Mac

If you're looking for something similar to the Alt key method used in Windows programs I'm afraid you're out of luck. That isn't supported on the Mac. The macOS does provide keyboard access to the Ribbon but IMO it's far more cumbersome than it's worth & provides excessively complete* access:

1. Press F6 [fn+F6] to access the Ribbon
2. Press Tab/Shift+Tab to navigate right/left to the tab you want
3. Press Space Bar to activate that tab
4. Repeatedly press Tab/Shift+Tab to get to the tool you want. Use ↓ if the tool has an expander [⌵]
5. When the selection is what you want press return

*By excessively complete I mean that the tabbing process toggles through all controls in the window including the Status Bar & eventually dumps you back into the body of the document. That ends the process.

A better process may be to use Control+F2 [fn+Control+F2] to access the menu bar. Tab/Shift+Tab or ←/→ to the menu name you want. (You also can press a letter of the menu name but there is no underscore so it's a bit of guesswork, especially if there is more than one menu name with the same first letter.) On the menu name either return, Space Bar or ↓ opens the menu then the same routine to select the command you want & press return*.*

IMO, the best alternative is to learn the direct keyboard shortcuts as well as creating custom keyboard shortcuts as needed.