Please make sure that the “Enable online payments” option is switched on (green). You can check this option in the card settings via "Finances” -> click on the card and then “Settings” is in the menu below the card. Once you tap on that, please scroll down until you find the options.
Please be aware that in some cases, the CVC of the card needs to be provided when paying online.
See also the article:
Where can I find the CVC for my card?
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