How to Allow/Activate External Booting on a Macbook

-First, after you are instructed to press "restart" while pressing the Commad+R keys, you will enter System Recovery Mode. Then go to the Utilities menu >> Startup Security Utility

-Then enter your device password

-Settings select "No Security" and "Allow boot from external"

-Then "Restart" while pressing the "Option+Command+R" keys

-And now the Macbook can boot externally :)