Can we ask an employee who is serving notice period to perform his task properly, especially the sales employee.

If a sales employee is serving his notice period either he stops doing sales or just dumps the material and later it ends in sales return, so can we inform the employee via mail that they need to achieve the target assigned and will it be legally accepted.

Yes, there is no illegality in asking the employee on notice period to do the work assigned to him without fail. Notice period is not a hiatus in the continuity of employment of a prospective resignee. If a prospective resignee gets himself involved in any misconduct during the notice period, he has to face the consequence of disciplinary action and the acceptance of his prospective resignation cannot exempt him from such an action.