SQL error trying to delete activities following CRM-20958
Start by checking that the civicrm_activity table does have a created_date column.  If not, maybe something went wrong with the upgrade process - which is a bigger issue.

If it does exist, do you have logging enabled?  When you change civicrm_activity, a trigger will update the related log_civicrm_activity table and if that does not have created_date then the original action will fail.  If that is case, you can disable and then re-enable logging at Administer &gt; System Settings &gt; Misc which should fix the problem.