Error when i try to purge against document.

You cannot purge the Document because it is used in the ECO item. This is the general Aras behaviour: If you have an Item that has a relationship to other items (it is either a property Item of other Itemtypes or related Item in a Relationship) you cannot delete it because it is used in its parent item. To be able to delete the Item you need to delete the relationship first. However if you already released the ECO you might not have enough permissions to lock/update the ECO. If the ECO is relased you also cannot "cancel" it anymore since the workflow is already closed. If you want to change the document content and not actually delete the Document from the system you may consider creating new revision. This can be accomplished either manually (by "version" item action on a Document) or by creating a new ECO/DCO and superseding the old Document.