Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Dear BeshoyAiad,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
According to your description, it seems you want to buy a subscription and you cannot add payment. Regarding this point, we kindly suggest you try use different browsers to check whether this issue occurs. If so, please refer Get support - Microsoft 365 admin | Microsoft Docs to contact our dedicated billing team, they will provide further help for you to buy the Microsoft 365 subscription.
Sincerely,
Tommy | Microsoft Community Moderator