You left out almost all the information required to debug this problem, such as:
- Are you running a calibre server or the server as part of a calibre GUI?
- Have you actually started the calibre server?
- What OS is calibre running under?
- What debugging steps have you taken?
Although you are not running Calibre Companion,
the CC Connection FAQ has a fair amount of information about what can go wrong. See Troubleshooting, steps 2 - 4.