Could not open input file: drush
The only way I have found to fix this problem is to change the default php used by the server. In other words, /usr/bin/php was originally a symlink to /usr/bin/php7.1, and changing it to a symlink to /usr/bin/php5.6 solved the problem.

But it appears that drush will always use the default php even if you install it with an alternative.