Originally posted by richyip
So is there any problem with the connect string?
Exactly.
jovery has posted the form of the connect descriptor you have to use.
The form you used is jdbc-like connect string and does not work in sqlplus.
Try:
sqlplus system/admin@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(Host=172.20.21.15)(Port=1521))(CONNECT_DATA=(SID=test)))