This error 'Person [number] is an allocated Party in Care package for' is caused due to either the source or target ID for a merge being the allocated party for another person, despite not having any finance data themselves.
There is no current way to merge records in this instance, 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 is an allocated party in a care package for another person.
The checks you could make on this is to:
Take the person IDs listed to you in the error box.
Go to the Person Summary screens and the Care Packages provided widget.
Check the services and elements summary screens for named allocated parties.
You may also be able to find the allocated party as a Payee on a payment cycle.
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 in this article.