Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
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This error indicates a possible billing related issue that's why it won't proceed purchasing additional storage. Few factors that you need to check like an expired payment method, failed renewal payment or canceled subscription in grace period.
I suggest that you sign in and have a look: https://account.microsoft.com/services? Look and search for, expired, past due, billing issue, renew, reactivate, or subscription.
You can check this using another browser as well.