What does the error "NOT FOUND - Resource can not be found at the specified address" mean?

Hello, I am trying to integrate Xero with Salesforce using MuleSoft Composer using HTTP connector.

I am able to GET and POST and update an existing invoice in Xero using Xero API Explorer and Postman. However, I am unable to do the same with MuleSoft Composer. There are no issues with GET.

But when I try POST, I get the error - "NOT FOUND - Resource can not be found at the specified address". Could someone help me understand what this error could mean please?