I just found I had this problem too, and it was caused by updating the module but not doing a Remove/Install so the new keys would be set up.
One new key you will need to set is the Merchant ID. It's a bit hard to find - login to BrainTree, go to the Dashboard, and click Settings->Processing. Then scroll to the bottom of the page. You'll see it under "Merchant Accounts" in the "Merchant Account ID" field.
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