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.