Mango refuses to refund me

Mango does not even deserve 1 star. We booked a flight early 2020 for 2 Adults and 2 children to fly to Johannesburg for my father in laws 70th birthday. Due to Covid all flights and travel was cancelled. Upon me requesting a refund, Mango refused saying they do not refund but will give me a voucher. I then informed them that we do not have any need in flying as this was a once off for a special occasion. They refuse to refund me. I do not need to fly nor does anyone want to buy this voucher from me as there is no trust in Mango Airlines. I cannot afford to lose this money and need Mango to refund it.