I have book the flight tickets from San Francisco to Kathmandu International Airport through Qatar airlines but due to lock down in Nepal I canceled the tickets and they send me the money voucher for future use unfortunately I have deleted it from my email I have temporary member ID number I have try to call to resend my voucher but they need the reference number or flight no which I don’t have.So how can I find the flight number?
Depending on when your original flight was booked the flight numbers could be the same as they were today i.e. QR 0738 and QR 0648.
If that doesn't work and you still remember the dates, there are websites that track historical flights based on route. See for example: https://www.flightradar24.com/data/flights/qr738 Most of those will require a paid subscription.
However, the flight number is not likely to help and I assume that Qatar will require either the booking reference and/or the ticket number. Both of those were included with the original booking confirmation and probably in the voucher as well. This is not publicly available data since it's specific to your booking and no one else business.
If you don't have any of these any more, you may out of luck. If Qatar can't find your booking by name & date or membership number, they will not give you another voucher. You need to proof somehow that you are indeed had an original booking and paid for it.