You may see this error message:
The selected date range is either a holiday, a non-working day, or both.
This means that the day requested is tracked as a non-working day for the employee. You can either change the date of the request, or you can adjust their working day profile if it's incorrect.
How to fix it:
Update the employee’s working day profile: Employees > Manage > Employee record > Job tab > Update > Change working day profile.
Change the time off request for a day that the employee is scheduled to work.
Advise the employee that they are not scheduled to work that day.