Very Bad service

My partner and I went to McDonald's Waterfall mall(Rustenburg) to buy food. I decided to redeem a deal which I had, Chicken fold over medium meal and chicken burger. After selecting everything and finalizing my order, it gave out a message to say the order is missing something but now when I press OK to go back it doesn't wana go back. We called a lady to come an assist who then asked if i scanned the code and chose everything that i needed which I rep**** yes, she then said she'll call the Manager as there's nothing she can do, she went to the back to call the manager. The MOD(Tebogo) came, asked me what's wrong and I rep**** her by saying the lady should have explained when she went to call her. She went to the self service machine and said she's gonna try reboot it, then she asked that i come to the POS so that she can redeem the deal(but now the 2 minutes was over and the deal was locked). I told her and she said there's nothing she can do, i will have to pay the normal price for both meals and should opt for Coke zero to which i rep**** asking how is it that i need to pay more when the problem is on their side and then again i only drink Orange juice(Dr's instructions).she said i should take water then i asked again, if i take water is it gonna be R69.90 as my deal amounted to that for both meals? Now my problem is, this is not the 1st time encountering such incident in both McDonald's in Rustenburg as it seems as if staff is not properly trained in terms of dealing with such issues, the MOD even says she can't call IT because it's gonna take time, and why am I forced to pay more when I had a deal that would save money? I am definitely not going back to either of this restaurants and I will tell other people how McDonald's is milking money out of customers.