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

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.