Can't boot from Big Sur installed on a USB
Get a new drive. That process is quite a bit longer than needed, but it’s correct. If you have errors with the install it’s likely these are the causes in order:

Bad USB storage
Bad download of the installer (very unlikely, but it happens)
Not following the guide
Failure of the Mac (more likely USB port or cable or hub)

Catalina is probably a safer bet for this activity for another couple months - Big Sur is still getting some edge cases patched up. So maybe get that downloading the next time you can let it run.