A 64015 Error is an indication that the configured connector for the payment method is missing one of the Parameter needed in the <Pay> verb.
The connector needs a specific Parameter in the <Pay> verb
The required parameters for the connector are listed in the "Description" section of the connector.
The parameters should be passed inside the <Pay> verb.
Example:
1`<Pay>`2<Parameter name="Parameter Name" value="Parameter Value" />3</Pay
MerchantAccountId is required to be passed via a <Parameter> noun for all Braintree <Pay> executions. This value can be found in the Braintree console by navigating to Settings (gear icon) -> Business.description as a parameter in <Pay>, you must also pass the following fields via separate <Parameter> nouns: name, kind, quantity, totalAmount, unitAmount. If you received this error you either left one of these fields out, or passed an invalid value for one of them. See Braintree Documentation for more information.