Hi Vijay,
You don't have to provide sales order and sales order item.
For account assignment type "Project Task" the determination of the sales order and sales order item happens automatically according to the rules that you find here:
- Work Center: Project Managment
- Work Center View: Not Invoiced Time and Expenses
- Mark a project and then press "Edit Time and Expenses"
- Perform the action "Edit Assignment Rules --> View All Rules"
Best regards
Otfried