Thank you for your quick reply.

I tried sending GET request, it gives me 200 response but within response body it give me the same error message as shown below.

GET

[Endpoint=callout:QuickBook_Online3/v3/company/9130XXXXXXXXXXXX/invoice/41556?minorversion=65&include=invoiceLink, Method=GET]

Response

{"Fault":{"Error":[{"Message":"An application error has occurred while processing your request","Detail":"System Failure Error: java.lang.NullPointerException","code":"10000","element":"SystemFailureError"}],"type":"SystemFault"},"time":"2024-02-22T04:36:14.617-08:00"}

Also I would like to ask do we need to turn on the Accept Credit Cards in Account and Settings for this to work?

Thank you for your help.