Why the Sync Error occurs
This error occurs when a vendor cannot sync to Oracle NetSuite because the "category" field is required in Oracle NetSuite. The category field for vendors isn't available in Bill.com.
How to Fix the Sync Error
Make sure the category field option isn't mandatory in your default vendor form in Oracle Netsuite. If it is, switch to the standard form as the preferred or to create a brand new custom form without selecting a non-default field as mandatory. Once a field has been checked as mandatory on a form, the sync will continue to call for it even if it's unchecked.
Alternatively, if the category field must be a required field in Oracle NetSuite, create vendors in Oracle NetSuite and sync them into Bill.com.