Required fields are missing [ REQUIRED_FIELD_MISSING]

This type of error message indicates that Salesforce has rejected the Insert / Update because a required field on the object is not populated.This could either be a standard Salesforce field, such as Name on Account, or Last Name on Contact, or a custom field which has the 'Required' attribute set to True.

An error message example would be the following:

We got an error back from SalesForce when trying to update or insert data in SalesForce: Error returned from SFDC on update: [fields: Industry_Category__c , fields: Industry_Subcategory__c , message: Required fields are missing: [Industry_Category__c, Industry_Subcategory__c] , statusCode: REQUIRED_FIELD_MISSING ],Data sent: sObjects: [type: Contact , Id: 0038e000008YLeNAAW , ContactNumber__c: A1234].

The above fields are required and they cannot be blank. You would need to update that information in our source system before transferring data to Salesforce.