Facing Error 'Missing expression' while trying to create a After insert trigger in PL/SQL
try this one

CREATE OR REPLACE TRIGGER T_Insert
AFTER INSERT ON tag

for each row
declare
  v_cnt number := 0;
BEGIN
  -- selection number of rows  
  -- where tag = :new.tag and AND brand_id = :new.brand_id
  -- the result will be stored in a variable v_cnt
  -- if there no rows matched the Count(1) will return 0 
  Select count(1)
    into v_cnt
    from my_tags
   where tag = :new.tag
     AND brand_id = :new.brand_id;

  -- check if some rows Exits
  -- than do an update
  -- else do an insert
  if v_cnt &gt; 0
  then
    UPDATE my_tags set count = count + 1
    -- if you want to update only one row
    WHERE tag=:new.tag AND brand_id=:new.brand_id 
    ;

  ELSE
    insert into my_tags
      (sr_no, tag, count, brand_id) values
      (:new.sr_no, :new.tag, 1, :new.brand_id);
  end if;

END;