Security settings do not allow this Mac to use an external startup disk

Steps to fix security settings do not allow this Mac to use an external startup disk.
Click on restart.
Press and hold Command_R as the MacBook restarts.
Hold the keys until the recovery screen appears.
Now click on utilities.
Click on Startup Security Utility.
Authenticate and enter your password.
Check Medium Security and Check Allow booting from external or removable media.
Click on the apple icon and click on restart.
Press and hold the option key to select and boot from your external media.