Vodafone's Free ChatGPT Go Campaign Blocked by Repeated Payment Errors

To take advantage of the "3 months of free ChatGPT Go" campaign on Vodafone's Yanımda app, I tried today to set up the subscription through the campaign link in the app. However, no matter what I did, I couldn't get past the payment step.
I'm using an Android device and Chrome browser. The campaign link redirects me to the ChatGPT Go subscription page, but at the payment step, even though I tried multiple times with different bank cards, I keep getting errors. I used both physical and virtual cards, Visa and Mastercard, trying around 10 times in total. My cards are enabled for online and international transactions, and I have sufficient balance; despite this, the system keeps showing warnings like "Your card was declined," "We couldn't verify your card," and "Payment method declined."
I don't think the problem is with the cards — I believe it's coming from the campaign or payment system itself. I want it checked whether my 3-month free ChatGPT Go benefit under the Vodafone Go campaign is actually applied to my line, and why I can't get past the payment step with any card. If necessary, I'd like a technical check done on my account and campaign enrollment to identify the issue, and I'm requesting the necessary support so I can take advantage of the campaign without any problems.
I'm waiting for an informative response that includes a solution.