I put this code into SQL Developer's Worksheet:

CREATE TRIGGER T_testDSNa
before INSERT
on testDSNa
referencing new as new
for each ROW
BEGIN
  SELECT S_testDSN.nextval INTO :NEW.SYSID FROM dual;
END;

I get this:

Error report -
ORA-00942: table or view does not exist
00942. 00000 -  "table or view does not exist"
*Cause:    
*Action:

Would anyone know why? This has worked for 3 previous tables until I tried to run the DDL to create a 4th. Alternatively, is there a better way to set up an autoincrementing PK?