Invalid voucher code, flights not booked

I booked my flights for May 2020, due to level 5, I opted for a travel voucher. Which is valid for 24 months. im trying to book the flights using the voucher no. It comes up with an error message saying the voucher no. Is invalid. The prices of the flights have now double as the prices change all the time till date no one has taken ownership of sorting this out as it's the customer who is always suffering due to this.