A declined card could be for any of the reasons here: https://learn.microsoft.com/en-us/marketplace/appsource-declined-card?source=recommendations
I would recommend you reach out to billing support as they can assist with more details why this is happening. Billing tickets are free.
If you have questions or need help, create a support request.
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola