KFC still peoples voucher

Bad service I had KFC voucheer went to the store to use it and they told me it's invalid and it's been 3weeks and handling my my complaint and now they tell me the is nothing they can do after I have spend my air time and data calling and emailing me ,Kfc is *****s because when I check the voucher it says it's redeem that means the staff have stole 120 voucher spoke to rude lady Nthuthuko and she has bad services