The first thing to check is the timesheet reference is correct, reach out to your line manager to confirm this.
If you still cannot see your timesheet, you will need to check if the vacancy has been actioned correctly. See the Vacancy journey article for more information on the steps of the vacancy as you may need to contact your agency or line manager to actions to be taken.
Please ensure that any outstanding tasks are completed as you will not be able to submit your timesheet until you have done this. You can see the Submitting your timesheet article for more guidance on this.