Hello. I don't see anything in the mapping field. Once you define the mapping field, it should work for you. 

The information at the bottom of this Docs page should help: https://docs.servicenow.com/bundle/london-hr-service-delivery/page/product/human-resources/concept/c_HRDocumentTemplates.html#PDFTemplateMapping

Some fields may not be appearing because they are not PDF Fillable fields. I see many federal forms have a box, but it is not a recognized form fillable field.

In additionI have seen cases where the property of the field (viewed in a PDF Editor application) will have special characters which impact the parsing. You can rename these fields within the PDF Editor to avoid those issues (that have special characters for the pdf field)

Regards,

Mike

ok, If it's not a prefillable form, what do I need to do? Find a software such as adobe to recreate the form as a prefillable form?

Yes, Adobe Pro can be used to convert the form into a fillable PDF. Just be sure the field names so not have special characters. I like to name each field descriptively. For example, first_name, last_name, etc. I find doing this makes it easier when I am mapping the fields later.