Why is a simple attempt to add an external reference no to an existing Sales Order via an import scenario failing?

I have set up many import scenarios before without problems.  Those have mainly been on Customer and Inventory Item records though.

I am trying to import line items into an existing sales order on SO.

This failed with an error so in narrowing down the cause I tried to do the simplest thing I could think of. Adding an external reference to the sales order.  I tested to see if I could do it manually and I can but if I try via the import scenario I get the error:

1 items have not been processed successfully. View name: Document.

The only thing in my import scenario is as below:

 The order no exists and the import scenario is using the same order number.

I can’t understand why this is failing.  Does anyone have any ideas?

Thanks,

Phil