Disgusting service

I went to McDonald's at Groblersdal today to buy my son's favourite meal for his birthday. We ended up leaving without ordering anything because the cashier there was very rude and irritable, number one they were short staffed, I asked why the service is so bad and do it so slow and he said people went to a party so they were there was no staff. so I wanted to redeem my voucher and he didn't know how to operate that he just referred me to the self-service of which it was also giving me some issues.i don't understand how *********** people can be left to run the business. I also wanted to the McFlurry which they didn't have.i asked for the manager he just looked at me and didn't answer me so I was not happy with the service it's so terrible please work on your service strategy