Transaction sent from Ariba Buying & Invoicing (P2P) is not sent to S/4 HANA or ERP due to below error triggered in SAP Integration Suite, Managed Gateway for Spend Management and SAP Business Network (Managed Gateway) Transaction Tracker:

Error reading XMLStreamReader: Unexpected character 'j' (code 106) in prolog; expected '<' at [row,col {unknown-source}]: [1,1]

Configuration issues in Cloud Connector.

Refer to Managed Gateway's Installation Guide to review the Cloud Connector configuration: Configuring SAP Cloud Connector

In Managed Gateway's Connectivity Tool, perform a connectivity test between Managed Gateway and S/4 HANA or ERP and if it fails one of the possible issues is with Cloud Connector.

For more information refer to Troubleshooting Managed Gateway for Spend&Network Connection issues Guided Answers

SAP Integration Suite Managed Gateway > Managed Gateway for Procurement