Hello Katie,
Thanks for reaching out. Any Contact inserted into Contacts table should have an Entity (Customer/Vendor/CustomerLead) associated with it. If you have not inserted an Entity already, please insert an Entity into Customer or Vendor table. Use the Name of that Entity when you insert Contacts into the Contacts table.
Example
Insert Customer with Name as 'Method Inc.'
Insert Contacts with Name as 'John Smith' and Entity as 'Method Inc.'
Also, in order to get the list of fields that are required or available to add/edit in any table you can use this call 'MethodAPIFieldListV2'. Our documentation should give you the list of fields necessary to call this endpoint.
https://www.method.me/wp-content/uploads/2018/12/MethodAPIDocumentationvV2.pdf