IF EXISTS() is semantically incorrect. EXISTS condition can be used only inside a SQL statement. So you might rewrite your pl/sql block as follows:
declare
l_exst number(1);
begin
select case
when exists(select ce.s_regno
from courseoffering co
join co_enrolment ce
on ce.co_id = co.co_id
where ce.s_regno=403
and ce.coe_completionstatus = 'C'
and ce.c_id = 803
and rownum = 1
)
then 1
else 0
end into l_exst
from dual;
if l_exst = 1
then
DBMS_OUTPUT.put_line('YES YOU CAN');
else
DBMS_OUTPUT.put_line('YOU CANNOT');
end if;
end;
Or you can simply use count function do determine the number of rows returned by the query, and rownum=1 predicate - you only need to know if a record exists:
declare
l_exst number;
begin
select count(*)
into l_exst
from courseoffering co
join co_enrolment ce
on ce.co_id = co.co_id
where ce.s_regno=403
and ce.coe_completionstatus = 'C'
and ce.c_id = 803
and rownum = 1;
if l_exst = 0
then
DBMS_OUTPUT.put_line('YOU CANNOT');
else
DBMS_OUTPUT.put_line('YES YOU CAN');
end if;
end;