PL/SQL Error Happening - triggers

Question

Select * from cat;
Attempting this

Answer 1

You double-fetch your cursor:
Your problem
[...]
OPEN UPDATETRIGGER;
FOR UT IN UPDATETRIGGER -- This performs fetching into UT
LOOP
FETCH UPDATETRIGGER -- Here You Fetch again..
[...]
What your loop should look like
CREATE OR REPLACE TRIGGER Display_Update_Message
BEFORE UPDATE
ON JOBS
FOR EACH ROW
WHEN ( (old.IsFilled != new.IsFilled) AND (new.isFilled = 'yes'))
DECLARE
CURSOR UPDATETRIGGER
IS
SELECT J.JobID JobID,
J.JobName JobName,
J.StopDate StopDate,
JS.LastName LastName,
JS.FirstName FirstName,
JS.Email Email
FROM JOBS J
FULL OUTER JOIN JOBAPPLICATIONS JA ON J.JobID = JA.JobID
FULL OUTER JOIN JOBSEEKERS JS ON JA.JSID = JS.JSID
WHERE J.JobID = :new.JobID;
JobID NUMBER (3);
JobName CHAR (30);
LastName CHAR (15);
FirstName CHAR (15);
Email CHAR (30);
StopDate DATE;
BEGIN
DBMS_OUTPUT.PUT_LINE (
'Seekers affected by closing job ' || JobID || ': ' || :new.JobName);
OPEN UPDATETRIGGER;
LOOP -- infinite loop
FETCH UPDATETRIGGER
INTO JobID,
JobName,
LastName,
FirstName,
Email,
StopDate;
EXIT WHEN UPDATETRIGGER%NOTFOUND; -- loop-breaker
:new.StopDate := SYSDATE;
DBMS_OUTPUT.PUT_LINE (
'--' || UT.LastName || ', ' || UT.FirstName || ' ' || UT.Email);
END LOOP;
CLOSE UPDATETRIGGER;
END;

Answer 2

You do not need to include the JOB table in the query. You have the information you require in the :NEW namespace. Not including the JOB table in the query will stop the ORA-04091 error.
Also you have two sets of cursor control statements. Choose one. I prefer the implicit cursor syntax because it is less typing (and marginally more efficient).
CREATE OR REPLACE TRIGGER Display_Update_Message
BEFORE UPDATE ON JOBS
FOR EACH ROW WHEN ((old.IsFilled != new.IsFilled) AND (new.isFilled = 'yes'))
BEGIN
DBMS_OUTPUT.PUT_LINE('Seekers affected by closing job '
|| :new.JobID || ': ' || :new.JobName);
FOR UT IN (SELECT JS.LastName
, JS.FirstName
, JS.Email Email
FROM JOBAPPLICATIONS JA
FULL OUTER JOIN JOBSEEKERS JS ON JA.JSID = JS.JSID
WHERE JA.JobID = :new.JobID )
LOOP
DBMS_OUTPUT.PUT_LINE('--' || UT.LastName || ', ' || UT.FirstName || ' ' || UT.Email);
END LOOP;
:new.StopDate := sysdate;
END;
/
Incidentally, I'm not sure why you have FULL OUTER JOIN in your cursor. I would have thought INNER JOIN was the correct solution. Surely you only want Job Seekers who have applied for the job you're closing? However, I have left that in because I don't know your business rules, and anyway I've changed enough of your code already :)