The IF statement has these forms in PL/SQL:
IF THEN
IF THEN ELSE
IF THEN ELSIF
You have used elseif which in terms of PL/SQL is wrong. That need to be replaced with ELSIF.
So your code should appear like this.
declare
var_number number;
begin
var_number := 10;
if var_number > 100 then
dbms_output.put_line(var_number ||' is greater than 100');
--elseif should be replaced with elsif
elsif var_number < 100 then
dbms_output.put_line(var_number ||' is less than 100');
else
dbms_output.put_line(var_number ||' is equal to 100');
end if;
end;