Downgrading Ubuntu 20.04 to 18.04

[Haochen Shi]

This bug occurs when I tried to downgrade my ubuntu 20 to 18 with the reinstall option in the installer.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.14
ProcVersionSignature: Ubuntu 5.3.0-28.30~18.04.1-generic 5.3.13
Uname: Linux 5.3.0-28-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
CasperVersion: 1.394.3
CurrentDesktop: ubuntu:GNOME
Date: Mon Jun 1 20:38:36 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed cdrom-detect/try-usb=true noprompt floppy.allowed_drive_mask=0 ignore_uuid boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 18.04.4 LTS "Bionic Beaver" - Release amd64 (20200203.1)
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)