Digitlogics — Presented upgrade voucher but has not been upgraded

This is to inform you that I mr gaffy garg is a platinum member of air Vistara.I m travelling on air Vistara flight UK 963 from delhi to mumbai on 9 February 2018.As a platinum member I have checked in through business class counter & presented upgrade voucher for next cabin class.At that time 1seat is available for the next cabin class according to your colleague name Mridul But he denied to upgrade me.The reason behind this he told me that sale is on now for that seat.so sir can u pls tell me that what does that voucher means????why I have spent lots of money for this???& else he told me that you have to wait for another 1 hour for upgrade but your coupon says 2 hours prior to departure if seat is available so anyone who have voucher can redeem it for the next cabin class.