If you cannot access Euphony after boot and you confirmed (by attaching monitor to Euphony machine) that you are being dropped to "emergency shell" or "emergency mode" because of some errors during boot you can try to:

Use earlier "Legacy boot" image

Create USB image using this file: http://euphonyimage-798b.kxcdn.com/euphony20190214v3.img.gz

On Windows, use Rufus program instead of EuphonyDownloader, and on other platforms use relevant instructructions from page: https://euphony-audio.com/installation/.

Be sure to extract euphony20190214v3.img from archive euphony20190214v3.img.gz and use euphony20190214v3.img file.

Too boot that image you must have "Legacy boot" enabled in BIOS.

This image only supports legacy boot but if you managed to boot from it - after you register and do "Complete installation" you will be able to update it to latest version (which will still boot).

Fix existing image

Another alternative is to try to fix existing USB disk which does not boot by using "gdisk" command utility.

Since Macs have the most problems with booting Euphony here is what you must do on Mac machine that you use to prepare Euphony USB image.

1. Install gdisk utility from: https://sourceforge.net/projects/gptfdisk/
2. Open Terminal end see which disk is your USB by typing: "diskutil list"
3. Use gdisk utility on USB device, i.e.: "sudo gdisk /dev/disk2" (if your USB is disk2)
4. On gdisk prompt line issue these 4 one-letter commands: x e w y (press Enter after every letter)

This should fix missing secondary GPT table that has to be at the very end of your USB disk and is missing in Euphony image.

Command syntax for gdisk is the same on Linux.