How Do I Install Kali Linux on a Linode

Sorry to hear that you're having trouble installing a custom distribution. It's a bit hard to tell what your exact issue is without seeing the specific error message that you're getting, but I think I have an idea.

The first method copying the disk uses Rescue Mode which runs the Finnix distribution. The wget method downloads to the Finnix RAM disk before transferring it to the installer volume. If you have less RAM than the size of your intended installer image, you can try to use the curl method mentioned in step number 2 in the Download and Install Image section of the guide you've linked. The curl method won't download the installer into the RAM all at once. Instead, it buffers in much smaller chunks.

I hope this helps! Feel free to reply with the exact error message you're getting if you'd like more help.

I also have problem installing Kali Linux on Linode, 2 CPUs and 4 GB Ram.
I think I've managed to "Create two configuration profiles and disable the options under Filesystem / Boot Helpers for each of them, as well as the Lassie shutdown watchdog under the Settings menu."
This is good as well:
"Installer profile
Label: Installer
Kernel: Direct Disk
/dev/sda: Boot disk image.
/dev/sdb: Installer disk image.
root / boot device: Standard /dev/sdb

Boot profile
Label: Boot
Kernel: Direct Disk
/dev/sda: Boot disk image.
root / boot device: Standard /dev/sda"

But I cannot download Kali installer file.
It's quite big, weighing over 4,3GB.
I'm using command:
curl https://cdimage.kali.org/kali-2021.2/kali-linux-2021.2-installer-amd64.iso | dd of=/dev/sda

As I understand it, this command downloads the file from url: https://cdimage.kali.org/kali-2021.2/kali-linux-2021.2-installer-amd64.iso
and saves it in the empty "Installer disk", right?

Is there any way that I can check if I've mounted the "Installer disk" correctly?
I've assigned 5120 MB to the Installer disk.
I don't think anything is downloading, because the output is shown in the exact moment of pressing enter key to execute the curl command.

Is there no way to just download the Kali ISO into some custom image dialog box in the Cloud Manager and install it from there?

I was having trouble with this as well, and I ended creating a third disk with the ext4 filesystem and mounting it to /mnt, this way I could download a large ISO file to that disk and write it to the Installer disk.