Why did I get API error code 10003?

PayPal presented API error code 10003.

Cause and Solution
This error can appear for many reasons. If you haven’t seen the API error codes page, it’s worth reviewing, but we’ll explain a few causes in this FAQ.

Causes for this error code depend on the API you’re using, but you can understand why you got the error by looking at the short message and long message in the API response.
Error Message Cause Solution
A transactionID is required. A transactionID was not specified. Include a valid transactionID in your API call.
Either transactionID or PayerID is required. No transactioniD or PayerID was
Include a valid transactionID or PayerID in your
API call.
InvoiceID is required. InvoiceID was not specified. Specify a valid InvoiceID.
Item name, amount and quantity are
required if item category is provided.
Missing Item name, amount, or
Check that your API call includes all required Item
One or more of these Giropay URLs is missing:
giropaySuccessURL, giropayCancelURL,
Missing Giropay URL Check that you specified a valid Giro URL.
OtherPaymentMethodID is missing. You specified no
Include OtherPaymentMethodID in your API call.
OtherPaymentMethodLabel is missing. You specified no
Include OtherPaymentMethodLabel in your API call.
OtherPaymentMethodType is missing. You specified no
Include OtherPaymentMethodType in your API call.
Start date is a required parameter. Your API call included no Start
date parameter.
Verify that you've correctly defined your Start date

For help resolving this issue or others, visit PayPal's Contact Customer Service page.