Oracle Enterprise Manager Installation
Why do you use the    -cluster - option ? 
Is you database repository part of a cluster ? 

Make sure first that you can connect to the database using the tns names.

Example : 

sqlplus system/password@'tns_identified'


Then to deconfig you em repo :
Try using this syntax :

 emca -deconfig dbcontrol db -repos drop -SYS_PWD oracle -SYSMAN_PWD oracle


For some nice articles on how you can prepare you Oracle Enterprise Grid Control Monitoring environment check the link