JCO3 Connection parameters

Hello,

In order to to call a function module using JCO, i am trying to connect to SAP server first. But the connection has failed and the console shows the following message :
Caused by: com.sap.conn.jco.JCoException: (103) JCO_ERROR_LOGON_FAILURE: Client 301 is not available in this system

Note: when i logon on the SAP GUI to do manually trasactions the client of my user is 301.

An other question, can I use the same user on both SAP GUI for manually transactions and calling a function module via JCO ?

Best Regards.