The electrum install to USB is messy and instructions are inaccurate. Running the electrum-4.5.5-x86_64.AppImage on the host computer doesn't install to the USB.
You can't install AppImage, since you just need to run it. But it's not portable by any means, since it'll create files on your home folder (
~./electrum) rather than on folder/drive where you run the AppImage.
A prompt window says Electrum wallet -> Wallet: default_wallet -> Choose... -> choose the USB path /media/hostComputerUsername/USB -> File name: there are no files, so the Open button is faded out?
I don't know what you expect. But based on your description, it seems you open a windows to choose wallet file. And by default, it's located on
~/.electrum/wallet.