To retrieve the latest version number, the CompleteFTP manager must contact the enterprisedt.com webserver. If access is blocked on your machine to this server, the manager will display this error. To diagnose the problem, please examine the manager's debug log file - it should tell you what the issue is. See here on how to obtain the log.
Apart from the URL being blocked, another possibility is that HTTPS is not working correctly. Our server requires the TLS 1.2 protocol to be used for HTTPS, and if the manager is being run on earlier versions of the .NET framework, TLS 1.2 will not be supported. You'll see a message in the manager log about TLS 1.2 not being supported. For example, the .NET 4.0 framework does not support TLS 1.2 (it only supports TLS 1.1). To get the update function to work again, the machine the manager is running on, must be running the .NET 4.5 or .NET 4.6 frameworks. In this case, you will need to update the .NET framework on the manager machine to either .NET 4.5 or .NET 4.6 (preferably the latter).