package grub-efi-amd64-signed 1.93.16+2.02-2ubuntu8.15 failed to install/upgrade: installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1

[Rich Dammkoehler]

/boot/vmlinuz-5.1.15-surface-linux-surface is unsigned.
E: Your kernels are not signed with a key known to your firmware. This system will fail to boot in a Secure Boot environment.
dpkg: error processing package grub-efi-amd64-signed (--configure):
 installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1

ProblemType: Package
DistroRelease: Ubuntu 18.04
Package: grub-efi-amd64-signed 1.93.16+2.02-2ubuntu8.15
Uname: Linux 5.1.15-surface-linux-surface x86_64
ApportVersion: 2.20.9-0ubuntu7.14
Architecture: amd64
Date: Tue May 12 12:10:22 2020
ErrorMessage: installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1
InstallationDate: Installed on 2018-02-19 (813 days ago)
InstallationMedia: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
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-signed
Title: package grub-efi-amd64-signed 1.93.16+2.02-2ubuntu8.15 failed to install/upgrade: installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1
UpgradeStatus: Upgraded to bionic on 2020-05-12 (0 days ago)