3829598 - Employee import error: "Missing required field - XXX. Record not processed for Employee_ID: XXXXX"

I ran an employee import file using the template I used before, However, when I ran the file this time I get an error: Missing required field - XXX. Record not processed for Employee_ID: XXXXX. What is a required field? And how can I fix it?

We run the employee import job, but some users were not created. I don't know which field is required field, and what does custom21 mean? , KBA , CON-CA-TLS-IEM-JOB , Scheduled Jobs , Problem