You can't book flights with a 20-21 hour transfer and then later ask for a (free) rebooking of the ticket with reference to a request for a 2 night stopover hotel. You need to book the ticket with a 2 night stopover from the outset if you want to use the TK stopover offer (which doesn't include transfers between airport and hotel).

With a 20-21 hour transfer at IST you are eligible for a free TK transit hotel - which includes transfer to/from the hotel - provided that there is no possible shorter connection time at IST than the flight you have chosen, e.g. an earlier flight from IST to your destination.