I have repeated the installation procedure and captured logging. flash-kernel exits with exit code 3:

2020-09-30 07:28:57,452 DEBUG subiquitycore.utils:83 arun_command ['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install'] exited with code 3
2020-09-30 07:28:57,453 ERROR root:39 finish: subiquity/InstallProgress/install/curtin_install: FAIL: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
subprocess.CalledProcessError: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
2020-09-30 07:28:57,452 DEBUG subiquitycore.utils:83 arun_command ['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install'] exited with code 3
2020-09-30 07:28:57,453 ERROR root:39 finish: subiquity/InstallProgress/install/curtin_install: FAIL: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
subprocess.CalledProcessError: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.


and that gives the following logging in curtin:

2020-09-30 07:19:55,807 DEBUG root:39 start: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/builtin/cmd-curthooks/installing-kernel: installing kernel
2020-09-30 07:28:56,459 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/builtin/cmd-curthooks/installing-kernel: SUCCESS: Status.FAIL
2020-09-30 07:28:56,480 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks/builtin: SUCCESS: Status.FAIL
2020-09-30 07:28:56,483 DEBUG root:39 finish: subiquity/InstallProgress/install/curtin_install/cmd-install/stage-curthooks: SUCCESS: Status.FAIL
2020-09-30 07:28:57,452 DEBUG subiquitycore.utils:83 arun_command ['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install'] exited with code 3
2020-09-30 07:28:57,453 DEBUG subiquity.lockfile:44 unlocking exclusive /run/subiquity/installing
2020-09-30 07:28:57,453 ERROR root:39 finish: subiquity/InstallProgress/install/curtin_install: FAIL: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
2020-09-30 07:28:57,455 ERROR subiquitycore.controller.installprogress:145 curtin_error
Traceback (most recent call last):
  File "/snap/subiquity/1972/lib/python3.6/site-packages/subiquity/controllers/installprogress.py", line 304, in install
    await self.curtin_install(context=context)
  File "/snap/subiquity/1972/lib/python3.6/site-packages/subiquitycore/context.py", line 142, in decorated_async
    return await meth(self, **kw)
  File "/snap/subiquity/1972/lib/python3.6/site-packages/subiquity/controllers/installprogress.py", line 281, in curtin_install
    self.logged_command(curtin_cmd), check=True)
  File "/snap/subiquity/1972/lib/python3.6/site-packages/subiquitycore/utils.py", line 85, in arun_command
    raise subprocess.CalledProcessError(proc.returncode, cmd)
subprocess.CalledProcessError: Command '['systemd-cat', '--level-prefix=false', '--identifier=curtin_log.2566', '/snap/subiquity/1972/usr/bin/python3', '-m', 'curtin', '--showtrace', '-c', '/var/log/installer/subiquity-curtin-install.conf', 'install']' returned non-zero exit status 3.
2020-09-30 07:28:57,458 DEBUG subiquity.core:470 generating crash report
2020-09-30 07:28:57,462 INFO subiquity.core:487 saving crash report 'install failed crashed with CalledProcessError' to /var/crash/1601450937.461412191.install_fail.crash
2020-09-30 07:28:57,465 DEBUG subiquity.core:511 show_error_report '1601450937.461412191.install_fail'

This time, the initrd image _is_ created:

root@ubuntu-server:/target/boot# ls -lrth
total 16M
drwxr-xr-x 2 root root 4.0K Jan  1  1970 efi
-rw-r--r-- 1 root root 253K Sep 10 10:12 config-5.4.0-48-generic
-rw------- 1 root root 5.3M Sep 10 10:12 System.map-5.4.0-48-generic
-rw------- 1 root root  11M Sep 10 10:36 vmlinuz-5.4.0-48-generic
drwx------ 2 root root  16K Sep 30 07:10 lost+found
drwxr-xr-x 2 root root 4.0K Sep 30 07:19 grub
lrwxrwxrwx 1 root root   24 Sep 30 07:28 vmlinuz.old -> vmlinuz-5.4.0-48-generic
lrwxrwxrwx 1 root root   24 Sep 30 07:28 vmlinuz -> vmlinuz-5.4.0-48-generic
lrwxrwxrwx 1 root root   27 Sep 30 07:28 initrd.img.old -> initrd.img-5.4.0-48-generic
lrwxrwxrwx 1 root root   27 Sep 30 07:28 initrd.img -> initrd.img-5.4.0-48-generic
drwxr-xr-x 4 root root 4.0K Sep 30 07:28 dtbs
lrwxrwxrwx 1 root root   51 Sep 30 07:28 dtb-5.4.0-48-generic -> dtbs/5.4.0-48-generic/marvell/armada-8040-mcbin.dtb
lrwxrwxrwx 1 root root   51 Sep 30 07:28 dtb -> dtbs/5.4.0-48-generic/marvell/armada-8040-mcbin.dtb