“Cannot connect to server” means just that. Your device is not able to connect to the remote server.
The server name is correct and works for me. That leaves a network error as likely source of the problem. If I had to guess, I’d say it’s IPv6-related. The fallback to IPv4 isn’t working properly in the current stable version of K-9 Mail (6.202). Try using the latest beta version. See How do I become a beta tester?