clean installation

It may help to know that you don't need a bootable USB drive to do a clean install of macOS. Instead, restart and hold Command–R to boot into macOS Recovery, then use Disk Utility to erase the disk—see step 2 in How to reinstall macOS from macOS Recovery.

If you cannot get the bootable installer to work, you can also try booting into Internet Recovery, erase the device from there, then install Catalina.

How to reinstall macOS from macOS Recovery - Apple Support

The most likely explanation is that something went wrong with the creation of the installer in the usb drive.

There are a few ways to do it. As you were told already, if done right it should appear as an option in Startup Disk in System Preferences, and also when booting with the Option key down.

Can you tell us the procedure you followed in trying to create the installer?

Also, please keep this in mind (since you said you were already running Catalina):

https://support.apple.com/en-us/HT208496

With the new filing system/Catalina, erase and install was changed a bit.

Hello, fbc1907.

Sorry to hear you're having this problem.

With the usb drive plugged into the MacBook, go to System Preferences > Startup Disk.

If you prepared the usb installer properly, then it should show up as a startup choice.

If not, then you'll need to redo the usb startup disk.

How to create a bootable installer for macOS - Apple Support.

Bad advice.