The Salesforce ID referenced in the error is the Household account record.

This error can occur if there is more than one record that is marked as Head of Household in the merge group.  For more information, please see this solution (BB750453).

If this is not the case, and there is only one record marked as Head of Household included in the merge, confirm that the Head of Household record is selected as the master record.  If a non-Head of Household record is selected as the master record, the merge will not complete.