What does the 'TxnID Cannot Be Linked: ...' error mean when applying a payment, and how can it be resolved?

Possible Reasons for the Error:

• Duplicate Customer in QuickBooks
  Check for duplicate customer profiles in QuickBooks and deactivate any redundant entries.
  
  

• Customer Profile Not Synced
  Verify the customer is synced in PortPro by opening the Organization Profile. If it’s not synced, click Sync to QuickBooks button, then select Save Changes.
  
  

• Payment Mismatch
  Verify that the invoice has not already been marked as Paid in QuickBooks.