SQL & PL/SQL

BEGIN
   MERGE INTO test123
      USING test12345
      ON (test123.c1 = test12345.c2 AND test123.c2 = test12345.c2)
      WHEN MATCHED THEN
         UPDATE
            SET test123.c3 = test12345.c2
      WHEN NOT MATCHED THEN
         INSERT (c1, c2, c3)
         VALUES (c1, c2, c2);
END;

error

ORA-06550: line 2, column 4:
PL/SQL: ORA-38101: Invalid column in the INSERT VALUES Clause: "TEST123"."C1"
ORA-06550: line 2, column 4:
PL/SQL: SQL Statement ignored