Troubleshooting the "No database connection configured for source plugin" error during a website migration

When trying to upgrade a D7 or D6 website to D8, it might be needed to declare the source database in your settings.php file, as explained in the Drupal documentation https://www.drupal.org/docs/upgrading-drupal/upgrade-using-drush

$databases['migrate']['default'] = [
  'database' => 'drupal7db',
  'username' => 'drupal7db',
  'password' => 'drupal7db',
  'prefix' => '',
  'host' => 'd7db',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
];

If you encounter an error message containing the following in your php-errors.log file

No database connection configured for source plugin...

... it might be that the place you are the $databases['migrate']['default'] snippet is not correct; it needs to be positioned after the Acquia Cloud required line.