package grub-efi-amd64 2.02-2ubuntu8.15 failed to install/upgrade: installed grub-efi-amd64 package post-installation script subprocess returned error exit status 1

[Eusebius Marcel]

Occcured when installing upgrades from Ubuntu 16.04 to 18.04.
1. 'lsb_release -rd' showed version 18.04
2. 'apt-cache policy pkgname' returns 'N: Unable to locate package pkgname'
3. Expected grub to be installed
4. Error message popup instead

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: grub-efi-amd64 2.02-2ubuntu8.15
Uname: Linux 4.20.17-042017-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
Date: Tue May 12 00:31:45 2020
ErrorMessage: installed grub-efi-amd64 package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2019-06-21 (324 days ago)
InstallationMedia: Ubuntu 16.04.6 LTS "Xenial Xerus" - Release amd64 (20190227)
ProcCmdLine: BOOT_IMAGE=/vmlinuz-4.20.17-042017-generic root=UUID=179aa94e-f69c-4165-9c45-bcd273911c02 ro quiet splash dis_ucode_ldr acpi=force vt.handoff=7
Python3Details: /usr/bin/python3.6, Python 3.6.9, python3-minimal, 3.6.7-1~18.04
PythonDetails: /usr/bin/python2.7, Python 2.7.17, python-minimal, 2.7.15~rc1-1
RelatedPackageVersions:
 dpkg 1.19.0.5ubuntu2.3
 apt 1.6.12
SourcePackage: grub2
Title: package grub-efi-amd64 2.02-2ubuntu8.15 failed to install/upgrade: installed grub-efi-amd64 package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to bionic on 2020-05-11 (0 days ago)