This error is caused by either the source or target ID for a merge having an invoice in the system, despite not having any finance data themselves.
There is no current way to merge records in this instance. If you are experiencing this issue, please see Merging duplicate person records for further information.
Where duplicate record merge is prevented and cannot be amended, you could consider restricting the record that is not to be used and adding a warning note on the to the effect of 'Do not use, this is a duplicate of case [case ID]'.
A merge cannot be performed if the source person has invoices.
Checks you can make
Take the person IDs listed to you in the error box.
On the Finance menu, click Find Invoice/Credit note.
Click the Advanced Find button at the bottom of the screen.
Find this person in the Service user details field.
If you still need assistance
If you would like to know what options you have available to you in the case where you are legitimately unable to merge, then please see the Options section on this article.