The draft payroll report is available once the payroll has been ran through the Draft payroll stage. This will show a draft or 'Dummy' payroll line for each worker processed and you will also be able to view a copy of the Payslip.
The report can be accessed by navigating to the Reports section on the left side menu and then selecting Payroll from the sub menu. From here you will see the screen below, where you can select either the Draft Payroll CIS or Draft Payroll PAYE/Umbrella.

This feature is very useful as a sense check if you a running large payrolls for 1000+ workers as it runs the payroll in isolation without sending any payslips/notifications or any submissions to HMRC or Pension providers. You can then export the draft payroll to .CSV and check totals etc before proceeding with full payroll.
If you encounter any errors with the draft payroll i.e. an incorrect rate amount or numbers of hours processed, you can cancel the draft payroll and re-run once the errors have been corrected.