I've been trying to activate several SIMs and had multiple credit cards rejected - super frustrating, especially for new users... Google searched the issue and I'm definitely not the only person running into this problem!
Have you tried the payment in a different browser, clearing your browser cahce, or by entering incognitio mode. Maybe your card declined the transactions for secruity reasons (I've seen a few cases about that). Also I've heard that Public Mobile has"issues" if the same credit card is used for 3 different accounts. I've seen some users say that they have had success with this though
Also make sure that the credit card information is exact! Card number, billing address, ect
I have had a friend experience the same issue that you're talking about. We tried muitlpe debit cards and were all rejected. The only way my friend got it to work was to Facebook message Public Mobile and then they were able to add the payment method manually from their end.
In the past I ran into problems with my card getting rejected on other sites when my address with the credit card did not match the address I was using with the purchase form.
