For incorrect data formatting - you will get an array of errors like below. The following example is for a bad expiration date format with a http response of 400 
{
    "message": "One or more validation failed.",
    "fluentValidatorErrors": [
        {
            "propertyName": "OneTimePaymentWithoutSubscriptionRequest.PaymentMethod",
            "errorMessage": "Expiration date needs to be in either one of the following format:  MMYY, MMYYYY, MM/YY, and MM/YYYY.",
            "attemptedValue": null,
            "customState": null,
            "severity": 0,
            "errorCode": null,
            "formattedMessagePlaceholderValues": null
        }
    ]
}
For a failure from the processor you will get a 200 response with a failed status and a message. To get the full rawResponse from the processor include: “includeRawProcessorResponse”: true, in the payload 
{
    "customerId": null,
    "invoiceId": 396247,
    "merchantInvoiceRefId": "ABC309500654810",
    "merchantPaymentMethodRefId": null,
    "networkTransactionId": "451831812015385",
    "invoiceStatus": "Noncollectable",
    "invoiceAttemptStatus": "Fail",
    "message": "Generic Decline",
    "amount": {
        "currency": "USD",
        "value": 1.03
    },
    "paymentMethodId": 6445,
    "paymentMethodTypeId": 1,
    "rawResponse": null
}
You can also run a GET on the invoiceID and get additional information: {{Api Root}}/api/Invoices/{{Active Invoice Id}} 
{
    "invoiceId": 292438,
    "parentInvoiceId": null,
    "customerId": null,
    "merchantInvoiceRefId": "ABC309500654810",
    "paymentMethod": {
        "paymentMethodId": 6446,
        "billingAddressId": 7528,
        "billingFirstName": "Joe",
        "billingLastName": "Smith",
        "merchantPaymentMethodRefId": null,
        "billingAddress": {
            "addressId": 7528,
            "addressLine1": "381 Forest Ave. Suite C",
            "addressLine2": null,
            "city": "Laguna Beach",
            "state": "CA",
            "postalCode": "92651",
            "phoneNumber": null,
            "email": null,
            "country": "US"
        },
        "paymentMethodAchDetails": null,
        "paymentMethodCreditCardDetails": {
            "binNumber": "445700",
            "paymentLast4Digit": "0005",
            "paymentExpirationDate": "0330"
        }
    },
    "invoiceStatus": "Noncollectable",
    "subtotal": 1.03,
    "tax": 0.00,
    "total": 1.03,
    "billingDate": "9/4/2024",
    "merchantLegalName": "[TEST] WP House (MD Adyen Route)",
    "merchantCustomerRefId": null,
    "customerFirstName": null,
    "customerLastName": null,
    "subscriptionId": null,
    "installmentId": null,
    "eligibilityFailReason": null,
    "merchantSubscriptionRefId": null,
    "networkTransactionId": null,
    "invoiceLineItems": [],
    "invoiceAttempts": [
        {
            "invoiceAttemptId": 134495,
            "amount": 1.03,
            "invoiceAttemptStatus": "Fail",
            "invoiceAttemptDate": "2024-09-04T18:35:39.95",
            "paymentProcessor": "WorldPay",
            "processorTransactionId": "84082941507574465",
            "responseCode": "110",
            "responseMessage": "Insufficient Funds",
            "paymentMethod": {
                "paymentMethodId": 6446,
                "billingAddressId": 7528,
                "billingAddress": {
                    "addressId": 7528,
                    "addressLine1": "381 Forest Ave. Suite C",
                    "addressLine2": null,
                    "city": "Laguna Beach",
                    "state": "CA",
                    "postalCode": "92651",
                    "phoneNumber": null,
                    "email": null,
                    "country": "US"
                },
                "billingFirstName": "Joe",
                "billingLastName": "Smith",
                "merchantPaymentMethodRefId": null,
                "paymentMethodAchDetails": null,
                "paymentMethodCreditCardDetails": {
                    "binNumber": "445700",
                    "paymentLast4Digit": "0005",
                    "paymentExpirationDate": "0330"
                }
            },
            "eligibilityCheckOrderCode": null
        }
    ]
}
Invoice Export