The 3rd Party app trying to integrate IFS with requires that certain messages be sent as a PATCH message.  Documentation for this is found here:

https://developer.paychex.com/documentation#tag/Worker/operation/updateWorker

Routing address for this which is set to use the PATCH operation.  The code to trigger the message is also set with a PATCH http method:

Other POST and GET messages to this service work fine, so it is not an authentication issue.  While testing this interface using Postman, the PATCH works fine.

When sending the same request as a POST, this gives the same result that IFS does when set as PATCH:

It appears that even when IFS is set to use PATCH in both the code and the Routing Address, it is actually sending it as a POST, which is being rejected by the endpoint. 

I have found similar post which has been closed without a conclusion.