Drush 8 installation issues
This method should work in almost all environments:

Browse https://github.com/drush-ops/drush/releases for the release you want. You can't use drush 9 with php 5.5, so you have to choose drush 8.

Download the selected drush version

wget https://github.com/drush-ops/drush/releases/download/8.1.3/drush.phar


and try to run this php runtime file:

/usr/local/php5.5/bin/php drush.phar status


The php path is an example. You only need it, if php does not pickup the right php version.

You can keep running drush like this, but it is more convenient to rename it to drush, make it executable with chmod +x drush and copy it to a location where your shell finds it as executable (a path defined in $PATH). This depends on that your environment allows you to execute phar files.