Payment to add time to my account is grayed out after I enter credit card information

The original post: /r/mullvadvpn by /u/geosc on 2024-08-17 14:08:42.

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Tried on all three of my devices, same issue. After I enter my CC information and try to click the Submit button, there is a red circle with a line through it and the submit option is grayed out.

How can I get my payment through? Do they not accept Amex or something?