If you have a contractor or employee who's eligible for withholding tax you will notice a warning on the employee’s pay details when you select the WT tax code.
This message is normal.
It will always display on the pay details when the WT tax code is selected, and won’t prevent you from saving the record.
Just make sure the employee is correctly registered for withholding tax with Inland Revenue, then disregard the message in the future.
For more information regarding Withholding tax please refer to the https://www.ird.govt.nz/income-tax/withholding-taxes page on the Inland Revenue website.