Random interest beening charged on 55 day interest free credit card account.

I called FNB credit card division from there I was transferred to Premier Banking to be informed that they have system issues. I have a 55 day interest free credit card account, for the 1st time in my life I have been charged interest on the 6th of October 2022 to the amounnt of R151.84. I alway ensure that my credit card is paid up on pay day. Kindly reverse this interest charged.