Using Oracle12C Client with Oracle11.g database
The document that has the client/server compatibility matrix is 207303.1 Client / Server / Interoperability Support Matrix For Different Oracle Versions. (Oracle support account required)

Oracle client version 12c is compatible with Oracle server version 11.2 (and below), so no your error isn't likely to come from that.

Check that the listener is indeed started on the database server, check that the database is up, and check that the database is registered with the listener.