No service or manager insisting you need to do her job

I went to Checkers Newmarket Alberton what a horrible experience. Manager and cashier rude don't understand customer service or there job. Especially the manager. I went to a till wanting to use my 10% extra savings in store. Swiped the card and continued scanning my items. Total was +/- R1544 when I asked the cashier to apply my discount she gave me a rude look and stated I will do it. The she advised me that my account or the customer is offline. I showed on my app that I have activated the 10% and stated she would call the manager. The parcel packer went to call the manager came to assist use after 10-15min with no explanation at the time as this store aircons was not working we died of heat waiting for her. At my surprise she arrived. I explained my problem and her response nearly had me in stitches that I can't believe they actually employed her as manager. Asking me if there is a contact number on the checkers app. Surely she should know it and not ask me to check as I don't work for the same company. I answered yes and gave her the number. She then insisted I call the call center. I responded stating I am the customer this is not my app or cashier system or my company I strongly advised her to contact she refused. After many arguments I left my groceries and left.