Gave up waiting for root device, ubuntu--vg-root doesnt exist

In certain Ubuntu versions (e.g. Xubuntu 18.10) this issue might be caused by an apt autoremove. Due to this bug apt will suggest to remove

cryptsetup cryptsetup-bin cryptsetup-initramfs cryptsetup-run dmeventd libdevmapper-event1.02.1 liblvm2app2.2 liblvm2cmd2.02 libreadline5 lvm2

which makes the system non-bootable (because the root partiation cannot be mounted and unencrypted using LVM).

If you are not using LVM and disk encryption this answer is probably not for you.

I was able to fix it by re-installing cryptsetup and lvm2 in a chroot environment: boot from a live USB stick, run commands below in a terminal, reboot.

# find root partition
sudo fdisk -l

# unencrypt partition
#   Note: replace /dev/nvme0n1p3 with your disk
#         replace "nvme0n1p3_crypt" with the correct name 
#         check by running this in chroot:
#         $ cat /etc/crypttab | cut -f1 -d " "
#         nvme0n1p3_crypt
sudo cryptsetup luksOpen /dev/nvme0n1p3 nvme0n1p3_crypt

# mount root partition
sudo vgscan 
sudo vgchange -ay
sudo mount /dev/mapper/xubuntu--vg-root /mnt

# prepare chroot environment
sudo mount /dev/nvme0n1p2 /mnt/boot/   # replace nvme0n1p2 with your boot partition!
sudo mount -o rbind /dev/ /mnt/dev/
sudo mount -t proc proc /mnt/proc/
sudo mount -t sysfs sys /mnt/sys/

# make dns available in chroot
sudo cp /etc/resolv.conf  /mnt/etc/resolv.conf 

# enter chroot
sudo chroot /mnt /bin/bash

# re-install missing packages
apt install cryptsetup lvm2

# re-generate  (this might be done also by apt in the step before, I'm not sure)
update-initramfs -u -k all

# Leave chroot environment - not sure if the following is really necessary...
exit
# Write buffers to disk
sudo sync
# Unmount file systems
sudo umount /mnt/sys
sudo umount /mnt/proc
sudo umount /mnt/boot

This Q&A helped me collecting the commands.

This Q&A is kind of related, but maybe not relevant for you anylonger if you are having this issue already.

Please add the outputs of lsblk -fs, fdisk /dev/sda with p flag and the report of grub repair to the question as pastebin links.

Based on searching, there seems to be similar questions on Ask Ubuntu that has no answer up to date here and Here one answer is given which explains about raid and ubuntu.

There seems to be several suggestions, some of which seem to work for some people.

1. From the initframs prompt try typing ls /dev/mapper and see if your root volume is listed. If it is not listed, try waiting 10 seconds and run ls again.

   If it is now listed, type exit and it should now find the root device and boot (taken from here

2. Edit the boot config by pressing e when grub alert to choose OS, and replace root=UUID-6500... by root=/dev/sdx where sdx is the boot partition. The problem here seems to be that the UUID is either wrong, or /dev/disk/by-UUID... does not exist(1)

3. Adding the parameter all_generic_ide to the end of the boot-line like kernel /boot/vmlinuz-2.6.27-7-generic root=UUID=43206294-74ef-434d-aca2-db74b4257590 ro quiet splash all_generic_ide seems to work for some people (2)
4. The Super User question here, seems to suggest the error is due to LVM. The solution is also given in the same which is to install lvm2.
5. The same Super User site also suggests booting using an older Kernel, if available. It seems to work for some people. Trying to boot using the Recovery option might work for some people
6. Post by nux_man777 here seems to suggest the error may be in the installation medium.

7. The error /dev/mapper/ubuntu-vg-root seems to be related to raid as described here.

   "In the installer summary screen right before the copy process starts, click the Advanced button. Change the boot partition (this is the MSDOS-style "parent" partition not the Linux partitions) to /dev/mapper/pdc_feddabdf (or whatever dmraid lists as your fakeraid partition) Make sure the checkbox is clicked to boot from this disk. note that the installer will modify grub2 to point to the correct logical partition /dev/mapper/pdc_feddabdf1 or whatever / is on."

   Detailed instructions regarding installation is given in the referred site. Please check there for more info on installation of grub.

8. Booting into a live installation media and updating ubuntu by changing root using chroot seems to work for some people (suggested by cpttripzz here). Instruction regarding chroot is here. You can also refer to this question at Unix & Linux. More details can be found at Arch Wiki and Gentoo Wiki.
9. Changing the SATA Controller to Native IDE from RAID and doing a fresh install seems to work for some people here and here
10. IDE cable or the hard disk may be bad (the same ubuntu forum as above page 40)

I had this problem and nothing on any posts here or elsewhere were able to help. Specifically in my case, I could see that /dev/mapper did not contain ubuntu--vg-root or anything else for that matter. This means that something either went wrong when LVM tried to mount/map the volumes OR something went wrong earlier in the boot process, and this error is just a catch-all symptom for any earlier problem.

The second of these was the case for me and only because of my reading about initramfs I was able to understand and diagnose the issue. This should be the first thing you do if you suspect something has gone wrong prior to LVM doing its thing.

In my case (which may not be the same as you but is worth documenting), I had full disk encryption (LUKS) enabled, and somehow the cryptsetup tools had been removed from initramfs, therefore I was not being prompted for the passphrase to unlock, and the drives were not accessible, which meant that /dev/sdaX could not be mounted, and therefore ubuntu--vg-root could not be mounted/mapped into /dev/mapper. If you try to run cryptsetup from the BusyBox prompt, youll know you have the same issue if thecryptsetup` cannot be found.

The solution was to boot from a LiveCD, unlock the drive manually with cryptsetup, chroot into the root filesystem, reinstall cryptsetup and call update-initramfs.