If USB is not visible in boot menu, probably you didnt flashed it successfully.
Which tools did you used ?
For example I tried with Rufus (iso mode) and it fails. Than I tried "dd" also.
With Balena Etcher I did it, but from second try..
Always first check sha256 for the .iso file. Than burn it on USB.
Those are first steps. If they are ok, than look on @Aravisian post