Can't install drush with composer When I run...

composer global require drush/drush:dev-master


...I get...

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Can only install one of: consolidation/output-formatters[3.1.12, 2.1.3].
    - Can only install one of: consolidation/output-formatters[3.1.12, 2.1.3].
    - Can only install one of: consolidation/output-formatters[3.1.12, 2.1.3].
    - drush/drush dev-master requires consolidation/output-formatters ^3.1.12 -&gt; satisfiable by consolidation/output-formatters[3.1.12].
    - Installation request for drush/drush dev-master -&gt; satisfiable by drush/drush[dev-master].
    - Installation request for consolidation/output-formatters (locked at 2.1.3) -&gt; satisfiable by consolidation/output-formatters[2.1.3].




My PHP version is 5.6.11.
The solution in my case was to run...

composer global update
composer global require drush/drush:dev-master


Previously, I had been running composer update but needed to run composer global update.