If you have had a trial previously, Azure will prevent you from using the same account and you will see that error message. See:
If this is not the case, you can try accessing the payment page from a different network in case the IP is being blocked for fraud prevention.
Your best bet here is to create a billing ticket. This is completely free here: create a support request
You can also call. The global support numbers are listed here: https://support.microsoft.com/en-us/topic/global-customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
Just select billing as issue type
You can mark it 'Accept Answer' and 'Upvote' if this helped you
(Please note: If you have Priority Community support please wait for a dedicated Microsoft support representative to assist you, as they have access to the necessary backend resources. If you have not yet opened a support case, we recommend reaching out through the support channel available under your subscription level.)
Regards,
Abiola