Old mutual voucher, manager refused without even trying

Hi can yo please assist, I went to Kfc at Steve Biko just about an hour ago with an old mutual voucher of 100 to purchase with, so immediately after mentioning that I want to use a voucher they said to each other that it does not work, and they came to me and said it does not work without even trying it and I asked them how do you know when you don’t even bother trying and wait for it to give you an error and they said it always does, I asked for the manager and she also stood there and said it’s not working and I asked her is she has ever escalated it as with other kfc we are able to be assist and she had no answer, that is the only Kfc I use on my way home from work. So how come they cannot assist and I had to end up using cash which I did not plan on using? Please assist because I was clearly given attitude and not customer service