I am trying to add the Person Notes field to a Marketo form but the field does not seem to be available to drag into the form editor.
Remember, all fields will pre-populate with data from the CRM system. Person Notes is a dangerous field to use in a form because sales people will often put text in there that they never intend the lead to see. We removed it form the form editor for this reason.
You can, if needed, add another field in your CRM and it will be available to use in Forms.