When you click on the "VNC Server" tab, you will see a window as shown here:

In this window, you can see if EchoVNC has detected whether a VNC Server is
running or not. If you started your VNC Server after you started EchoVNC,
you may need to click the Check Again button.

There is also a button here to prevent EchoVNC connections to your VNC
Server. You may want to use this feature if you are using EchoVNC to connect
to your clients' PC's, but don't necessarily want your clients to be able to
connect back to your PC.