Open "Simatic Shell" on both the server and the client. Ensure they are on the same terminal bus and can "see" each other in the shell window. Fix B: Configuration File (.pnc) Issues
If your download and installation are perfect but the viewer still won't connect, your firewall is likely blocking the traffic. wincc viewer rt download fix
Ensure the Client Viewer version matches the Server's Service Pack. Open "Simatic Shell" on both the server and the client