I tried to reproduce the issue and found it is an "unsupport platform" for the tool flash-kernel


ubuntu@ubuntu:~$ dpkg -l *1044*
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                                Version                        Architecture                   Description
+++-===================================================-==============================-==============================-============================================================================================================
ii  linux-headers-4.15.0-1044-raspi2                    4.15.0-1044.47                 armhf                          Linux kernel headers for version 4.15.0 on ARM (hard float) SMP
ii  linux-image-4.15.0-1044-raspi2                      4.15.0-1044.47                 armhf                          Linux kernel image for version 4.15.0 on ARM (hard float) SMP
ii  linux-modules-4.15.0-1044-raspi2                    4.15.0-1044.47                 armhf                          Linux kernel extra modules for version 4.15.0 on ARM (hard float) SMP
ii  linux-raspi2-headers-4.15.0-1044                    4.15.0-1044.47                 armhf                          Header files related to Linux kernel version 4.15.0
ubuntu@ubuntu:~$ sudo apt-get install --reinstall linux-headers-4.15.0-1044-raspi2 linux-image-4.15.0-1044-raspi2 linux-modules-4.15.0-1044-raspi2 linux-raspi2-headers-4.15.0-1044
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 4 reinstalled, 0 to remove and 0 not upgraded.
1 not fully installed or removed.
Need to get 0 B/44.6 MB of archives.
After this operation, 0 B of additional disk space will be used.
(Reading database ... 113356 files and directories currently installed.)
Preparing to unpack .../linux-image-4.15.0-1044-raspi2_4.15.0-1044.47_armhf.deb ...
Unpacking linux-image-4.15.0-1044-raspi2 (4.15.0-1044.47) over (4.15.0-1044.47) ...
Preparing to unpack .../linux-headers-4.15.0-1044-raspi2_4.15.0-1044.47_armhf.deb ...
Unpacking linux-headers-4.15.0-1044-raspi2 (4.15.0-1044.47) over (4.15.0-1044.47) ...
Preparing to unpack .../linux-modules-4.15.0-1044-raspi2_4.15.0-1044.47_armhf.deb ...
Unpacking linux-modules-4.15.0-1044-raspi2 (4.15.0-1044.47) over (4.15.0-1044.47) ...
Preparing to unpack .../linux-raspi2-headers-4.15.0-1044_4.15.0-1044.47_armhf.deb ...
Unpacking linux-raspi2-headers-4.15.0-1044 (4.15.0-1044.47) over (4.15.0-1044.47) ...
Setting up linux-raspi2-headers-4.15.0-1044 (4.15.0-1044.47) ...
Setting up linux-modules-4.15.0-1044-raspi2 (4.15.0-1044.47) ...
Setting up flash-kernel (3.90ubuntu3.18.04.2) ...
Setting up linux-headers-4.15.0-1044-raspi2 (4.15.0-1044.47) ...
Setting up linux-image-4.15.0-1044-raspi2 (4.15.0-1044.47) ...
Processing triggers for linux-image-4.15.0-1044-raspi2 (4.15.0-1044.47) ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-4.15.0-1044-raspi2
flash-kernel: deferring update (trigger activated)
/etc/kernel/postinst.d/zz-flash-kernel:
flash-kernel: deferring update (trigger activated)
Processing triggers for flash-kernel (3.90ubuntu3.18.04.2) ...
Unsupported platform.
dpkg: error processing package flash-kernel (--configure):
 installed flash-kernel package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 flash-kernel
E: Sub-process /usr/bin/dpkg returned an error code (1)

ubuntu@ubuntu:~$ flash-kernel 
Unsupported platform.

ubuntu@ubuntu:~$ sudo flash-kernel 
Unsupported platform.