Sales Rep Field is not available under Mass Update fields for Transaction

If we would like to Mass Update the sales rep's name for various transactions (Opportunity, Estimate, Sales Order, Invoice and Cash Sale), the Sales Rep field may be missing under Lists > Mass Updates > General Updates > select a respective Transaction (Opportunity, Estimate, Sales Order, Invoice or Cash Sale) > Mass Update Fields. 

This is caused by the Team Selling feature enabled. To disable the feature, perform the following steps:

1. Navigate to Setup > Company > Enable Features.
2. Click the CRM tab.
3. Under the Sales section, uncheck Team Selling.
4. Click Save.

Now the problem is solved.