Refund on our bus tickets

I am not happy .We bought tickets over year ago to visit family in Namibia and we paid over R4000 for the tickets. We were to leave the first week of april. And the president call a national lock down due to Covid and the borders was closed . They issued us vouchers that are valued for 3 years. A year is already has gone by and still it is not possible for us to go to Namibia. And now that greyhound has close down due to this pandimic we are scared that we will loose our money or our the tickets. We try to get a refund this past year but they will not allow and they enply that we have to use the voucher and they will not give back our money. All that we want is our money back that is all . What can we do to get our money back