Answer: 

The transaction was deleted after it exported to Xero or the you have an existing transaction in Xero with the exact same number in same closed off period. This is why Xero shows the transaction as "received" but cannot provide an active "link" to that transaction within Xero.

Additional Information:

• Restore the deleted transaction if it was voided.
• Amend the number of the existing transaction to something unique then try re-exporting from Unleashed.
• Do not not modify the journals in any shape or form in Xero.
• When are Duplicate Cost Bills Created in Xero?