Can I understand more clearly:
You create a bootable USB stick, boot to 18.2, but there is a failure during 18.2 installation.
I have had a similar problem, when I downloaded the installation ISO, but the download was corrupted (damaged). It appeared to install correctly, but booting produced an 'insert bootable disc' error message.
Re-downloading the ISO and verifying the checksum allowed me to proceed and install successfully (on 4 machines and a virtual box image so far), including on machines that previously ran 17.3 or 18.1.
Note: the above were Cinnamon installs, not KDE, but that should not matter.
https://linuxmint.com/verify.php