Configuring Embedded Search 
The search function in the Help Center is based on the Embedded Search solution from SAP NetWeaver, which uses the TREX search engine. To make Embedded Search available for the Help Center search, you must establish an RFC connection between your SAP system and the TREX server.
Create the RFC connection to the TREX server and establish the connection.
More information: Creating a Connection Between Embedded Search and TREX