Hi normalina. You'll have to provide some more detail here. Which version of Koha are you on? And not least: Which Z39.50 server are you trying to connect to? Please list all the information about it that you have under Home › Administration › Z39.50 servers. Or were you trying to connect to Koha's own Z39.50 server?
*** This bug has been marked as a duplicate of bug 9242 ***