First you have to enable the Trace forwareder on the panel

Then you have to start a tool on the host where you have installed TIA with Unified (change IP to your Panel IP):

"c:\Program Files\Siemens\Automation\WinCCUnified\bin\RTILtraceTool.exe" -mode receiver -host 192.168.210.128 -tcp

Then you can start the trace viewer on the PC:

"C:\Program Files\Siemens\Automation\WinCCUnified\bin\RTILtraceViewer.exe"