Unable to add payment method to my Apple Account

I have always had Samsung phones and my kids have been on me about getting an iPhone. I did, and I have had nothing but trouble out of it. It’s an IPhone 15, and today I was ready to throw it in the trash. Started by trying to pay my JC Penney payment for online (thru synchrony bank) and it wouldn’t let me “add my payment source”. Thinking their website might be the problem, I called their automatic payment line and took care of it (using the same payment source that it wouldn’t let me add online). So tonight I got on my phone to pay my Best Buy payment (thru Citi Bank) and got the same message while trying to add the checking account info that connected to my debit card that I tried to use to pay JC Penney this morning. It’s not my checking account that’s the problem either!! Why can’t I add a new payment source to any of my bills in this iPhone?? I tried accessing the websites thru safari and google. I gave up and called their automatic payment line at Best Buy and paid it over the phone….using the same debit card that my iPhone wouldn’t let me add my online. WTH???

[Re-Titled By Moderator]