Apt-get is missing on the latest firmware 2.21.111

What should I do in order to install apt-get on the latest update ?

Just a general warning. Performing an apt-get on the My Cloud has a nasty tendency to brick the My Cloud or cause other issues. One can do a subforum search to find several threads where people have performed an apt-get and bricked their My Cloud (primarily the older v4.x versions).

WD MyCloud Gen1 have software with non-standart memory page size (64kb vs normal 4kb), so its not compatabile with software from official debian distro.
I make my own distr with software, recompiled for this firmware: Repository with software worked on v4 firmware

WD MyCloud Gen2 dont have any OS inside, just Busybox + few tools. So there is no Apt or other package managers. But if you need something - use Chroot’ed Debian and install anything you want inside.

I take exception to the statement that gen2 does not have an OS just Busybox. They do have a Linux kernel. Gen2 systems boot up Linux just like a gen1. Busybox is a program that has been compiled with a lot of the normal Linux utility programs into one code file. It then creates symbolic links for these programs to Busybox. It ends up using less disk space. Not sure why they had to switch from a normal Linux system
to Busybox. By normal I know that it uses 64k rather than 4k.

RAC

My firmware version is 2.21.111. It is MyCloud 8TB.

Firmware version 2.21.111 is version that uses busybox.

RAC

As stated earlier; you do not want to use apt-get unless you really understand how the MyCloud is built, and know what you’re doing. And having used a standard Debian linux system before does not cut it; the MyCloud isn’t a standard Debian linux box.