Is this your only organisation on this account? and is this your only account? Just trying to see if you might be using an API key from another Org or Account.
Yes,this is my only organisation on this account and this is my only account. I think there are some bugs about the usage limit system
Do you get this immediately, or after a short while? Because there is a limit of three transactions per minute until there is a payment method entered into billing.
i just met the same problem. Did you solve it? 
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