Hi,

You can do something like this, in the constrains check the leave type and given dates and raise validation error if the entered date is a future date.

class HRLeave(models.Model):
    _inherit = 'hr.leave'

    @api.constrains('request_date_from', 'request_date_to')
    def check_sick_dates(self):
        # set current date to the variable current date
        if self.holiday_status_id.name == 'Sick Leaves':
            if self.request_date_from > current_date or self.request_date_to > current_date:
                raise ValidationError(_("Sick Leaves is not allowed for Future Dates.. !"))

Thanks