Dear Shaheryar,

Try This:

from odoo.exceptions import  ValidationError

    @api.multi
    @api.constrains('end_date', 'start_date')
    def date_constrains(self):

              for rec in self:

                      if rec.end_date < rec.start_date:

                           raise ValidationError(_('Sorry, End Date Must be greater Than Start Date...'))

I hope I help you...

Hello Ayman,

Thanks for replying. How can I do this on Odoo Studio, the online Saas version?

I tried to use the following in Automated actions but that did not work.

def date_constrains(self):

              for rec in self:

                      if rec.end_date < rec.start_date:

                           raise ValidationError(_('Sorry, End Date Must be greater Than Start Date...'))