There might be the situation where that port used by the application to be already used by another version running on your PC or you're trying to install the application without Administrator access. If an application needs access to system settings to open a PORT or something related and it's launched without Administrator access, then it will display an error like the one above.
Luckily, you can see what is the application using the 20121 port. Download and run TCPVIew: https://technet.microsoft.com/en-us/sysinternals/tcpview.aspx
Once you locate the application with the problem, force close it and install iCMSPro again.