You're right. We can't add a column for "Sales Rep" to the sales reports in QuickBooks Online. This is because this function is only available in QuickBooks Desktop.
If you're using the Class Tracking feature for sales rep tracking, use the Class column instead. Also, you can add a Created by column in the Sales by Customer Detail report. It will tell which invoices were created by a certain user.
Let me know if you need anything else. Have a great day.