The sales rep field is missing in the edit and create modes of sales orders in NetSuite, but it is visible in view mode. One possible reason for this could be that the team selling feature is enabled in your NetSuite account. When the team selling feature is enabled in NetSuite, the sales rep field will not be available in edit and create modes. Additionally, a sales team subtab is added to the sales order record, where sales details can be entered and managed collectively by the sales team members involved in the transaction.
Check if the feature is enabled in your account by navigating to Setup > Company > Enable Features > CRM subtab > Team Selling. If it is enabled, disable it. This will allow you to see the sales rep field in both edit and create modes.