The first thing to check is the timesheet reference is correct, we suggest for them to reach out to their line manager to confirm this.
If they still cannot see the timesheet, they 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 to check that it has been actioned correctly.
The candidate will need to ensure that any outstanding tasks are completed as they will not be able to submit their timesheet until they have done this. They can visit the Submitting your timesheet article for more guidance on this.