If you've upgraded to Drupal 8.5.x and above, and have used composer to install drush 9, you will find that commands that use Acquia Site Aliases no longer work. Drush 9 is a complete rewrite of drush, and the alias file format is now completely different.
When you are logged in to cloud.acquia.com, you can also use this script for all of your sites and extract the archive into $HOME
$ tar -C $HOME -xf $HOME/Downloads/acquia-cloud.drush-aliases.tar.gz
Log in to https://accounts.acquia.com/ and visit your profile then choose Credentials the link to download the drush alias is under the Drush Integration codeblock.
See https://www.drush.org/ for more information about using drush 9