I'm working through the workbook to get familar with everything but keep coming up with an issue in the flow section. I am getting this error message. Can anyone help me please?

Sorry, an error occurred in the flow.

For help, provide your flow administrator with the following information: UPSERT --- UPSERT FAILED --- ERRORS : (REQUIRED_FIELD_MISSING) Required fields are missing: [Invoice__c] --- for SFDC record with ID : null,

Thank you