Acquia cloud installing a module: private key file
The solution was to forget about Acquia and start using cPanel.

In here, was as straightforward as creating a subdomain, taking advantage of Softaculous scripts by installing in that location the 8.4.3 version of Drupal.

Once installation was complete (which was a matter of seconds) and I've logged in as admin, installing a module was never easier! 

See for yourself:


Tiago