Thank you for your question.
Please create a SwissPass Login (www.swisspass.ch) and afterward log in into the SBB App or the www.sbb.ch website using your credentials.
Save your credit card as a payment method and tray the purchase again: usually it is best to buy tickets with the SwissPass Login instead of as a guest.
If this does not work, please check with your bank.
If they say everything is alright, check here: SBB Mobile | SBB help & contact
In the FAQ there are many helpful tips and at the end of the page you can find a contact form as well, in case you could not solve the problem.
I wish you a great day