Invoice Billable Time > The Rate field is greyed out and cannot be changed

This happens when the Combine TimeItems on Invoices preference is enabled. To be able to enter different ratesfor same items entered by same/or different employees, disable/unmark theCombine Time Items on Invoices preference.   

1. Navigate to Setup > Accounting > InvoicingPreferences

2. Remove the checkmark on CombineTime Items on Invoices

3. Click Save