Unable to view bootable start up USB from System Preference -> Start Up Disk

I want to downgrade from Big Sur to High Sierra. According to instruction I got from MacWorld (https://www.macworld.co.uk/how-to/bootable-mac-installer-3575875/#make%20installer)

Following the instruction, I erased a USB (steps 1 - 6), but I was not able to implement Step 7 "Choose GUID Partition Map as the Scheme. " because there was no such option. I managed to place the downloaded High Sierra into the USB (see Image 1 below).

I skipped step 7 and finished the rest until step 20. I was not able to implement Step 21 "Your Mac will display the Startup Manager, click on your external drive to select that as the startup disk. Your Mac will start up in Recovery Mode," because I did not see the bootable USB after restarting my Macbook Air while pressing the Option key.

So, I went back to the the upper directory of the article (https://www.macworld.co.uk/how-to/bootable-mac-installer-3575875/). Under "How to create a bootable installer for macOS" section. However, the USB is not recognized in System Preference (see Image 2 below).

I am at loss as to what I missed. Pls help.

Image 1

Image 2