Upgrade from 4.6.20 to 4.6.21 fails with fatal error (sqlfile not found)
I would recommend you grab this file https://github.com/civicrm/civicrm-core/blob/4.6/CRM/Upgrade/Incremental/sql/4.6.21.mysql.tpl and then save it into same path within your civicrm dir i.e. in CRM/Upgrade/Incremental/sql/

Once you have done that re-load your pre-upgrade database and run the upgrade again. 

Should be all good then