So here is my Ubuntu version:

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.2 LTS
Release:        18.04
Codename:       bionic

I'm trying to run the following command: pyenv install 3.6.2 but i get the error:

Command 'pyenv' not found, did you mean:
  command 'pyvenv' from deb python3-venv
  command 'p7env' from deb libnss3-tools 

I've searched and this post (Ubuntu 14.04 - Python 3.4 - pyenv: command Not Found) states Ubuntu 14.04 and below use Python 2 be default so one has to use virtualenv instead, but why does my 18.04 Ubuntu not recognize the command?

First see if you have the curl already installed in your machine using the command:

$ curl --version

If you don't have, install the curl using:

$ sudo apt-get install curl

After that install the pyenv using the command:

$curl https://pyenv.run | bash

And after installation update your bashrc adding the lines:

export PATH="~/.pyenv/bin:$PATH"
eval "$(pyenv init -)"
eval "$(pyenv virtualenv-init -)"

Finally Reload the bashrc:

$ source ~/.bashrc

I think will work fine after that. If you installed the pyenv before, look up at you bashrc to confirm if you added the lines above and reload the bashrc again.