This message indicates that the Free Textfield 3 is not set up in the accounting system for the document to be posted in Twinfield. You can set this up per journal. 

Example: for purchase invoice you can find these settings in Twinfield when you go to
Settings > Company settings > Transaction types > purchase invoice.
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Name: Basecone
​Link: copy & paste the link below to this field (see image below)
​https://secure.basecone.com/lookup?GUID=$freetext3$
(Control this link when you paste this into the field that it's the same)

For more information, see article: Settings to retrieve invoices in Twinfield
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