Hi @ vd128, please try setting the DISPLAY variable to localhost:0.0 after you connect to the remote machine. If Bash shell is used in the server then run following command:
export DISPLAY=localhost:0.0
Next try running the xclock command. Hope that helps.
If not then most likely something is wrong with your server configuration. Please refer to this article to ensure the X11 forwarding is setup correctly on the remote machine. Note that you would have to have administrator permissions to modify the server configuration.