Make sure you use the same validator. To void a ticket you have to use the same validator, which the ticket was issued with. 

You can void the ticket until 23:59 ( 0UTC) of the day of ticket issue

Yes, you have to use the following codes in the requests to switch between use cases:

• For ETK void — code "702"
• For EMD void — code "Y"

If the ticket is issued at the day of departure, can I void it after departure or only until?

If not, until what time before we need to void it?

 The main thing isn't to allow a NO-SHOW (NS) — if the passenger failed to show for the flight before the check-in closing and notify the carrier of cancelling the carriage

You can void the ticket after departure until 23:59 of the current day (If the ticket is not used), but it is better to void the ticket before departure.