apt-get: command not found

If apt is not found so Linux is CentOS which is RedHat-based
RedHat installs use yum not apt-get

Some of the solutions that will help for solving this issue :=
===============================================================

1. https://stackoverflow.com/questions/5231160/sudo-apt-get-command-not-found

2. https://www.psychz.net/client/question/en/apt-get-command-not-found.html#:~:text=If%20the%20command%20returns%20no,%2Fetc%2Fapt%2Fsources.&text=In%20the%20mirror%2C%20go%20to,file%20suitable%20for%20your%20architecture.

3. https://askubuntu.com/questions/538676/etc-apt-sources-list-not-found

There can be 2 issues :=

1. Your are trying the command in machine that does not support apt-get command
because apt-get is suitable for Linux based Ubuntu machines; for MAC, try
apt-get equivalent such as Brew

2. The other issue can be that your installation was not completed properly So

The short answer:
-----------------
Re-install Ubuntu from a Live CD or USB.

The long version:
-----------------
The long version would be a waste of your time: your system will never
be clean, but if you insist you could try:

==> Copying everything (missing) except for the /home folder from the Live
CD/USB to your HDD.

OR

==> Do a re-install/repair over the broken system again with the Live
CD / USB stick.

OR

==> Download the deb file for apt-get and install as explained on above posts.
I would definitely go for a fresh new install as there are so many things to
do and so little time.