British Airways Comair cancelled flight, no refund or voucher

I had booked to fly to Zimbabwe on the 29 March 2020. On the 24 March I received a letter stating that my flight reference OWJU3Z has been cancelled. I was then taken to a web site site where I could choose a refund or voucher. I selected refund. I then received a letter on 7 April stating sorry for the inconvenience but I do not qualify for a voucher. Basically British Airways Comair has ****** my R5k