Hi, I followed the detailed step-by-step suggestion by @bpierre, with two changes: (1) Using the UEFI boot mode (which is the default) as suggested by @dell-mario l, and (2) ignoring the keyboard layout-related issues (since I have an QWERT keyboard). Below is the list of steps.
Result: Encryption worked (!), However, "Dell Linux Assistant" now aborts with an error, and "Dell Recovery" has different menus than prior to factory status. See below for copy-paste of the error message. @dell-mario l, could you or others kindly advise? I guess that I will not get Dell updates this way. Best would be to have the automatic dell notification as before. Alternatively, I could install a dell support tool, or check manually on a Dell support page for updates. Lastly, I don't mind of re-doing this process, with required changes to make it work.
Steps to enable disk-encryption:
1 - Download the dell-recovery_1.60_all.deb Github following this link:
2 - Mount the recovery partition of your embed SSD disk, for example using the following command: sudo mount /dev/nvme0n1p2 /mnt/
3 - Copy the downloaded file into the /debs/main folder of the mounted recovery partition: sudo cp dell-recovery_1.60_all.deb /mnt/debs/main/
4 - Unmount the recovery SSD partition: sudo umount /mnt/
5 - Create an ISO recovery image using the dell recovery tool
6 - Insert a blank USB flash memory and create a bootable recovery key using the ubuntu "Startup Disk Creator"
7 - Reboot your computer, press F12 to show the startup menu and select the USB key to boot in UEFI mode
8 - At the beginning of the setup, there should be two options: "Restore Entire HD" and "Restore Only Linux" choose the second (i.e 'only Linux').
9. Choose "Erase Disk and Install Ubuntu" and mark "Encrypt the new Ubuntu", this will auto-mark also the LVM option.
10 - When asked for an encryption password, pick any password, assuming you have a QWERT keyboard
11 - Keep the USB recovery key to a safe place
12 - Run sudo apt-get update && sudo apt-get upgrade to keep the system up to date.
Here are the partitions, after this process (by GParted):
/dev/nvme0n1p1 EFI, FAT32, /boot/efi, 512MB total, 7MB used
/dev/nvme0n1p2 EXT4, /boot, 732MB total, 257MB used
/dev/nvme0n1p3, [Encrypted], 475G total, 475G used
The error on "Dell Linux Assistant" is: "Exception: The name com.dell.SummerPalace" was not provided by any .service files."
For "Dell Recovery" there is only one option "Build OS Media". Before this process, in the factory default, there was also the option of creating an ISO. Clicking the "build OS media" leads to a request to provide a Base OS ISO.