Message
Invoice with same number X exists in QuickBooks Online, link these?
Cause
This is not a error. The connector has detected that an invoice exists in QuickBooks Online with the same number as the one it is attempting to import.
Resolution
If these are the same invoices, you should check off the button to link them. If they are not, you should go into QuickBooks Online and change the number of the other invoice or delete it so the invoice can be imported.