Acquia Dev Desktop and Drush broken with syntax is incorrect
I was also facing the same issue on windows 10 with Acquia Dev Desktop and configured existing Drupal 8 project from a client.
Following three things what I did worked for me.
I simply:

Removed directory vendor/drush
Removed directory vendor/bin
Removed line &quot;drush/drush&quot;: &quot;^8.1&quot; from composer.json which is located on docroot directory.

Now, default drush is working for me as you can see the default drush version at C:\Program Files (x86)\DevDesktop\tools\composer.json.
Hopefully, this will help someone else.