Drush dl not working with PHP 7 and ubuntu 16.04
Sounds like you managed to install/configure PHP without the simplexml extension.

Install the php7.0-xml package with apt-get as follows. 

sudo apt-get install php-xml


Also check the UI installer, if you're missing this, then you're likely also missing a bunch of other extensions link mbstring, gd and so on to install/use Drupal.