NodeBB installation: SyntaxError: Unexpected token?

https://docs.nodebb.org/installing/os/ubuntu/

I am following the NodeBB documentation(link attached above) to setup nodeBB locally on my machine - However, I am stuck at "Installing nodeBB" step with the below error.

$ ./nodebb start
/home/pravallika/nodebb/node_modules/commander/lib/command.js:380
    enableOrNameAndArgs = enableOrNameAndArgs ?? 'help [command]';
                                               ^

SyntaxError: Unexpected token ?
    at Module._compile (internal/modules/cjs/loader.js:723:23)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
    at Module.load (internal/modules/cjs/loader.js:653:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
    at Function.Module._load (internal/modules/cjs/loader.js:585:3)
    at Module.require (internal/modules/cjs/loader.js:692:17)
    at require (internal/modules/cjs/helpers.js:25:18)
    at Object.<anonymous> (/home/pravallika/nodebb/node_modules/commander/index.js:2:21)
    at Module._compile (internal/modules/cjs/loader.js:778:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)

Browsed for solutions, I have seen suggestions related to version of nodeJS so I checked it too,I don't think that's the issue. Not sure of how to proceed.

Specs & installations:

$  lsb_release -a  
No LSB modules are available. 
Distributor ID: Ubuntu 
Description:    Ubuntu 20.04.6 
LTS Release:    20.04 
Codename:   focal

$  node -v
v10.19.0

$  npm -v
6.14.4