>i an facing problem with the trigger. plz give me a solution how i can do this
Please clarify EXACTLY what "problem" you face.
BTW, you have a typo error in posted code.
>var:=chcek1(:new.room_no);
not as above but as below
var:=check1(:new.room_no);
>dbms_output.put_line('The room is not available');
line above makes NO sense, because PL/SQL runs deep inside RDBMS engine & no user will ever see it.