Couldn't redeem my voucher

Poor service I got and the cashier wasn't friendly I want to Burger King Rivonia Boulevard to purchase a 1 King Size Combo and mind you I was there on the 20th of May 2023. I completed the survey, so that on my next purchase I can get the free whopper Jr When I got there to purchase and redeem I was told that my voucher is invalid and I wasn't given a reason why it's invalid another thing I have used it today was the first time I was using it