Link to source property will be set to ‘true’ when you create a document type using some source like XSD, or XML, or Flatfile schema or Broker document type… Once it is linked, you cannot edit the document even if you have locked the document type object…
If you still try to edit, the property will change to ‘false’.
If you want to modify the structure of that document, you have to edit ‘link to source’ to ‘false’ and then you will be able to make any change…
Reason for failure in your case:
When you tried to pull the document from broker, it will try to refresh the structure that is already in place. As this property is ‘true’, the structure of this document type cannot be updated. Hence this error.
Note: This property is visible only through Designer not through Developer.