Tried the below steps & it worked.
1. Delete (or backup) existing generated "config_oam.jar" file from archive location (-archiveLoc) before re-running.
2. In the same command prompt set also CLASSPATH like below:
export CLASSPATH=<OAM_HOME>/oam/server/pswdservice/lib/pswdmgmt-api.jar:$CLASSPATH
3. Re-run copyConfig command.