magento2 – Getting “Vendor autoload is not found.” error even the composer install succeeded

When i open Magento in browser i got

Autoload error

Vendor autoload is not found. Please run ‘composer install’ under application root directory.

even i have run

composer update
composer install

on the root folder and it says

Nothing to install, update or remove

Is there somewhere I can dig to figure out why the autoload is still failing?