snags at "apt install linux-headers-generic/master linux-image-generic/master"

Thank you. I made the changes you recommended but it still threw an error and the resulting ISO did not work. It's intended to work around an issue with Dell Inspiron 7490 laptop which has no provision for switching from RST to AHCI, and so the Ubuntu installer cannot see the primary drive. That is still the case with the resulting ISO, which is intended to incorporate code from the Endless OS installer, which does work. Secure boot and disabled during the install attempt.

ERROR: couldn't connect to zsys daemon: connection error: desc = "transport: Error while dialing dial unix /run/zsysd.sock: connect: no such file or directory"

So it looks like a problem external to Cubic. I thank you for your time and effort.

Matt,

I noticed the following path in the instructions you linked to.

    squashfs/etc/apt/sources.list.d/endlessOS.list

There is no "squashfs" path when you are in the Cubic terminal environment.

May be the person who posted those instructions meant "/etc/apt/sources.list.d/endlessOS.list" instead (without the squashfs directory)?

The instructions you linked to are basically installing a different Linux kernel.
I presume this kernel was compiled by Endless OS.
I assume this kernel has been modified to change some parameters ('RAID on' and Disk Mode).

Here are the correct instructions (just four simple commands):

    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    wget -q https://deb.endlessos.org/keys/eos-pub-archive-key.asc -O- | apt-key add -
    echo "deb [arch=amd64] https://deb.endlessos.org/debian master endless" | tee -a /etc/apt/sources.list.d/endlessOS.list
    apt update
    apt install linux-headers-generic/master linux-image-generic/master
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

On the Kernel tab of Cubic's Options page, be sure to select the newer kernel you just installed (5.11.0-19).

I was able to install the kernel.
(I did get a warning about "Possible missing firmware". You may want to install the missing firmware if you need it).

I did not test boot the generated ISO. I'll leave that testing up to you.

Breakthrough! Thanks so much!

I am still left with one problem: The installation fails with
"The 'grub-efi-amd64-signed' package failed to install into /target/. Without the GRUB boot loader, the installed system will not Boot."
And on exiting the installer crashed.
I used all the defaults and minimum install ran the install with fast boot off. Can you help me find a clue to fix this?
(I am dual-booting with windows)
You've done your part exceptionally well, so if this is not in your wheelhouse just let me know, We'll still be friends!

Again, many thanks for getting me this far!
~matt

You can see what the problem is if you try to manually install this package in Cubic:

    apt install grub-efi-amd64-signed

    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable
    distribution that some required packages have not yet been created
    or been moved out of Incoming.,
    The following information may help to resolve the situation:

    The following packages have unmet dependencies:
    grub-efi-amd64-signed : Depends: grub-efi-amd64-bin (= 2.04-1ubuntu44.2) but 2.04+dev245.7421863-28bem1 is to be installed
    E: Unable to correct problems, you have held broken packages.
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

In order to install grub-efi-amd64-signed Ubuntu needs grub-efi-amd64-bin *VERSION* 2.04-1ubuntu44.2.
However, version is 2.04+dev245.7421863-28bem1 is available.

Try installing grub-efi-amd64-signed manually in Cubic's Terminal environment, BEFORE you add the PPA for your custom (Endless OS) Linux kernel.

Execute the following in a NEW customization project in Cubic:

    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    apt update
    apt install grub-efi-amd64-signed

    wget -q https://deb.endlessos.org/keys/eos-pub-archive-key.asc -O- | apt-key add -
    echo "deb [arch=amd64] https://deb.endlessos.org/debian master endless" | tee -a /etc/apt/sources.list.d/endlessOS.list
    apt update
    apt install linux-headers-generic/master linux-image-generic/master

    rm /etc/apt/sources.list.d/endlessOS.list
    apt update
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Also notice in the last two commands above, we remove the endlessOS PPA and refresh (update) the sources list. This is to stop Ubuntu from trying to install additional packages from the EndlessOS PPA. Once you've installed the EndlessOS customized Linux kernel, you probably only want packages (such as "grub-efi-amd64-signed") from the *official* Ubuntu repositories, rather than from the EndlessOS reposotories. This may help avoid additional errors like, "Unable to correct problems, you have held broken packages".

Thanks Cubic PPA, that solved my question.