For prebid-server the example shows json='{"pageOpts": {"member": 958,"invCode": "ast_guaranteed_prios_1","..., especially the member value. Without setting this, requests to https://ib.adnxs.com/ut/v3
fail with
{"error":"Could not find valid member in request"}
What is pageOpts.member? Which value should it be set to?
The example you gave is not for prebid-server, it is for accessing Appnexus directly. In order to setup prebid server you should follow their AMP instructions here. Also, to debug the auction on the client side you can add: &debug=1 to the url that sends the request to prebid-server, it generally is prebid.adnxs.com/pbs/v1/openrtb2/amp?, however that will be different if you use your own hosting.