Unable to get Angular CLI version, though all the requirements are installed?

I would recommend you to remove your NodeJS installation completely and re-install NodeJS through NVM. This way you can easily switch between multiple NodeJS versions and the other benefit of that for each active NodeJS version you may able to install specific Angular CLI version.

e.g:

nvm install 11  
nvm install 12
nvm alias default 12  
nvm use 11  
npm install @angular/cli -g

• Mac/Linux: https://github.com/nvm-sh/nvm
• Windows: https://github.com/coreybutler/nvm-windows

Also see some details on Angular CLI version here

P.S
If you where using brew, to uninstall NodeJS follow this steps:

brew uninstall node; 
# or `brew uninstall --force node` which removes all versions
brew cleanup;
rm -f /usr/local/bin/npm /usr/local/lib/dtrace/node.d;
rm -rf ~/.npm /usr/local/bin/node;

For node v8 install version of angular cli 7.3

npm i @angular/[email protected]

As, the Angular CLI requires a minimum Node.js version of either v10.13 or v12.0.

STEP 1: Install v12.x

nvm install 12

STEP 2: Use v12.x

nvm use 12

STEP 3: Install Angular CLI

npm install @angular/cli -g

STEP 4: Check version, verify its installed

ng --version

sample image of output: https://i.stack.imgur.com/9jr15.png

Angular CLI requires a current, active LTS, or maintenance LTS version of Node (https://angular.io/guide/setup-local#nodejs). which version 11 isn't.

Node.JS 11 is no longer supported by the Node.Js team as after six months, odd-numbered releases (9, 11, etc.) become unsupported. More info: https://nodejs.org/en/about/releases

Solution : Upgrade Node version to 12+