Command pm-enable needs a higher bootstrap level to run
Is your settings.php file in sites/all/default?  If not, you'll need to add the --uri option.  If your settings.php file is in sites/all/foo.com, then add --uri=foo.com.

drush --uri=foo.com en seo_checklist


Or, change your working directory to the same folder that contains settings.php.

Or, define a site alias @foo that defines 'root' and 'uri', and then use:

drush @foo en seo_checklist