I just had this problem. Found the solution here - https://github.com/braintree/braintree-web-drop-in/issues/161 (second post by crookedneighbor). The problem is that your PayPal sandbox account that is receiving the payments is set to reject the currency of the Braintree sandbox. My Braintree sandbox currency is set (hard wired) to Euros, and my PayPal sandbox account was set to only accept British Pound. Go here https://www.sandbox.paypal.com/businessmanage/preferences/payments - login with your PayPal sandbox username and password. Under Allow payments sent to me in a currency I don't hold: select Yes, accept them and convert to <your currency>. Additional charges apply
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