Setting Up Alerts to Remind People

I hope my question is straight forward enough, but I am trying to set up date reminders in Smartsheet when a field is not filled out to remind the submitter that we have not received all information.

For instance, I built a Smartsheet to track Purchase Orders. The user is supposed to fill out when they receive the Purchase Order. How can I send a reminder each day to that user until that cell is filled out?