Problem:
The MassLOOP button requires you to specify a Contact Id for the &contactfield= parameter but Person Accounts leverage Account Ids rather than Contact Ids.
Solution:
If you are generating the document from the Person Account record, use PersonContactId .
Example: &contactfield=PersonContactId
If you are generating the document from a custom object, reference the Person Account look-up field beforePersonContactId .
Example: &contactid=customfield__r.PersonContactId