network encryption not available in the net manager

For some Oracle Database 11GR2 Standard Edition (windows 2008R2) i have not the window to access to the network encryption. If I try to put it directly with a "SQLNET.ENCRYPTION_SERVER = ACCEPTED" alone it doesn't work : no crypto algorithm in the banner columns of V$SESSION_CONNECT_INFO .If I add "SQLNET.ENCRYPTION_TYPES_SERVER= (RC4_256)" , it blocks all connections with this error:   TNS-12649: Unknown encryption or data integrity algorithm.

it seems to mean that the setup of the the database does not include security components which are usually available in the 11gr2 release or that something else interacts  negatively.