Comment 111 for bug 1835660

I struggled with the same issue on a fresh Kubuntu 20.04 installation (Lenovo Thinkpad P52s).

From the Grub / Advanced options / Recovery mode / root terminal I was able to remount the / as rw (rather than ro)

Changing 'lz4' to 'gzip' as per https://forums.linuxmint.com/viewtopic.php?f=46&t=323152
and running update-initramfs -c -k $(uname -r) I had a new issue: the fstab entry UUID=3e757667-25352354... was not resolved.

To solve this I had to:
manually changed the fstab entry from UUID=... to /dev/nvme0n1p2
change COMPRESS=lz4 to gzim in /etc/initramfs-tools/initramfs.conf
update-initramfs as per above
reboot