This is caused by the volunteer record being associated with a household record. This violates the CK_VOLUNTEER_VALIDCONSTITUENTTYPE constraint and it should not be possible for records like this to be in CRM. They are usually inserted by conversion or early day sync setup users.
When the merge happens, it tries to move them to the retained household record, which violates the check constraint, so you get this error.
The solution is to either move volunteer records to a new constituent that is not a household or delete the volunteer records. If you are unable to do this in CRM, contact TA Support for assistance.
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