Please follow this support entry:
https://support.endlessos.org/en/known-issues/no-such-device-ostree
To make things short:
- Boot with a Live USB Stick like the one you used to install Endless OS
- Open any Terminal app and make yourself root with
sudo bash
- Execute the following commands then:
fsck -f -y /dev/disk/by-label/ostree
mount /dev/disk/by-label/ostree /mnt
touch /mnt/.file
umount /mnt
- Power down the computer, remove the USB Stick and boot again