How do I resolve this installation error? I am getting the following error while installing Drush with composer require drush/drush.


  Using version ^8.1 for drush/drush
  ./composer.json has been updated
  Loading composer repositories with package information
  Updating dependencies (including require-dev)
  Your requirements could not be resolved to an installable set of packages.      
  
  Problem 1
  - drush/drush 8.1.0 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.1 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.2 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.3 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.4 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.5 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.6 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.7 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.8 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - drush/drush 8.1.9 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
  - Conclusion: don't install drush/drush 8.1.12
  - Conclusion: don't install drush/drush 8.1.11
  - Conclusion: don't install drush/drush 8.1.10
  - Conclusion: don't install drush/drush 8.x-dev
  - Installation request for drupal/drupal [No version set (parsed as 1.0.0)] -> satisfiable by drupal/drupal [No version set (parsed as 1.0.0)].
  - Conclusion: remove phpdocumentor/reflection-docblock 3.1.1
  - Installation request for drush/drush ^8.1 -> satisfiable by drush/drush [8.1.0, 8.1.1, 8.1.10, 8.1.11, 8.1.12, 8.1.2, 8.1.3,
  8.1.4, 8.1.5, 8.1.6, 8.1.7, 8.1.8, 8.1.9, 8.8.x-dev, 8.x-dev].
  - Conclusion: don't install phpdocumentor/reflection-docblock 3.1.1
  - drush/drush 8.8.x-dev requires phpdocumentor/reflection-docblock ^2.0 -> satisfiable by phpdocumentor/reflection-docblock [2.0.0,
  2.0.0a1, 2.0.0a2, 2.0.0a3, 2.0.1, 2.0.2, 2.0.3, 2.0.4, 2.0.5].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.0, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.0a1, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.0a2, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.0a3, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.1, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.2, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.3, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.4, 3.1.1].
  - Can only install one of: phpdocumentor/reflection-docblock [2.0.5, 3.1.1].
  - Installation request for phpdocumentor/reflection-docblock (locked at 3.1.1) -> satisfiable by phpdocumentor/reflection-docblock [3.1.1].
  
  Installation failed, reverting ./composer.json to its original
  content.
Had the same problem started from D8.4beta1.

Solved by switching to Drush 9 with:

composer require drush/drush:^9