Creating Entity with an invalid parent.

When you create an email, letter or phone call through SDK (Plugin, Workflow or from your website or Silverlight application) you might get the error 'Creating Entity with an invalid parent. Entity: Email'

The reason behind this is your regarding object does not support activity association.

All you have to do is navigate the entity from Customisation and tick Activities in the 'Communication & Collaboration' section and publish the entity.

Hope this will help.