OpenKM has many interesting features, but requires some configuration process to show its full potential.
Forum rules: Please, before asking something see the documentation wiki or use the search feature of the forum. And remember we don't have a crystal ball or mental readers, so if you post about an issue tell us which OpenKM are you using and also the browser and operating system version. For more info read How to Report Bugs Effectively.
After OpenKM started you should have a soffice.bin task under tomcat, if you do not have it you have missed something sure. Check your catalina.log file into tomcatXXX/logs folder, there's shown the exact reason of your error.
Take a look at catalina.log for complete stack trace error.
Also, I suggest trying from clean OS + browser to be sure there's nothing in the computer installed like antivirus, AD killer what might be causing some issue.