There is a timesheet rule that can be configured to warn or prevent employees from submitting timesheets that are incomplete.
To find this rule, first navigate to the timesheet profile that you'd like to update by navigating to: Company Settings > Profiles/Policies > Timesheets
Once you open up the timesheet profile you'd like to edit, navigate to the View/Edit Rules button:
Select Add Rule at the top right, and because you'd like this action to trigger when the employee submits their timesheet, you'll need to search under the category of "On Submit".
After selecting the "Incomplete Timesheet" rule, you'll be prompted to select one of two options, Warning or Error. Configuring this rule with a warning will let the employee know that their timesheet is incomplete, but allow them to submit it. Configuring it with an error will prevent them from submitting an incomplete timesheet.