I have struggled lot to resolve "Drush was not able to start (bootstrap) the Drupal database..." issue in Drupal 8. Finally, after burning the midnight oil I have found solution as below.
My laptop configuration: PHP 7, WAMP 3, Windows 7 and 10
The main reason of such issue is PATH variable. You need to add your MySQL and PHP path in system PATH variable. To check, you should close your command shell and restart. It will start using updated PATH variable
In my system, I am having WAMP 3. Path for PHP and MySQL path as below
Credit goes to https://www.drupal.org/node/1393232