Existing target database must be a Oracle Rdb database if WITH
  CHECKING is specified.

  Explanation:  You used the TO EXISTING clause specifying a
  database that is not an Oracle Rdb database and did not specify
  the WITH NO CHECKING clause.  The default is WITH CHECKING,
  however, this can only be used with Oracle Rdb databases.

  User Action:  Use the WITH NO CHECKING clause in the transfer
  definition.