If a user access a Sales Order (in View Mode), the Sales Rep field is visible in the Main Body of the Transaction. However, if they Edit the Transaction, the Sales Rep field disappears from the Main Body.
This is caused by the Team Selling feature wherein it hides the Sales Rep field from the Main Body so that the user can assign a sales rep in the Sales Team tab on the Transaction.
Here are the steps in disabling the Team Selling feature:
1. As Administrator, navigate to Setup > Company > Enable Features.
2. In CRM tab, set Team Selling= F.
3. Click Save.