Employees are able to set unavailabilities via the Employee Self Service Portal, provided this has been enabled via the Payroll settings > Employee portal settings page.
There may be scenarios where an employee is unable to save an unavailability because it falls within the threshold set on the Payroll Settings -> Employee Portal Access page. That is, you can set how many days before an employee must enter their unavailability.
In the case above, if you enter 14 days as the cut-off date, employees will need to enter their unavailability at least 14 days before the unavailability falls due.
If they attempt to enter an availability in the period before the unavailability falls due, they will get an error message.