DECLINED AUTHORIZATION FEES FOR UNUSED CARD

So I took out a credit card in June with FNB in the hopes of building my credit score as I had compromised it, I’ve been getting smses to my phone “ insufficient credit for my transaction ” when my card isnt being used or haven’t been for a couple of days. I am legit chilling at home and my card hasn’t been used for the past 6 days and I got a declined authorization fee, when there’s money in my card, tell me how sway??? This has been happening for the past few months and I shrugged it cos I thought they were sending the smses by mistake until I check my transactions kanti this whole time I’m being charged. I need an explanation as to what’s going on because my score is now being compromised, I am the only one who has access to this credit card and for the times i didn’t have money in my card, why would I use a card that doesn’t have money ??? Like I need answers because this is compromising my credit and I would rather close this credit card and take it with another bank if I can’t rely on you guys to the right thing. I didn’t bother calling customer service because they are useless especially the credit card customer line ! Terrible customer service, my dog is better of taking those calls. I need someone from complaints to contact me, provide me with a sound answer and rectify