here is my tables and trigger

drop table room;
create table room(
room_no integer not null,
catagori varchar(20) not null,
quality varchar(20) not null,
price integer not null,
primary key(room_no)
);

set serveroutput on;
create or replace procedure room_av(
roomno room.room_no%type,
catagoris room.catagori%type,
qua room.quality%type,
pri room.price%type) is
begin
insert into room (room_no,catagori,quality,price) values (roomno,catagoris,qua,pri);
commit;
end room_av;
/
begin
room_av(102,'single','2nd_class',200);
room_av(103,'single','2nd_class',200);
room_av(104,'single','2nd_class',200);
room_av(105,'single','2nd_class',200);
room_av(202,'single','1st_class',300);
room_av(203,'single','1st_class',300);
room_av(204,'single','1st_class',300);
room_av(205,'single','1st_class',300);
room_av(302,'double','2nd_class',250);
room_av(303,'double','2nd_class',250);
room_av(304,'double','2nd_class',250);
room_av(305,'double','2nd_class',250);
room_av(402,'double','1st_class',400);
room_av(403,'double','1st_class',400);
room_av(404,'double','1st_class',400);
room_av(405,'double','1st_class',400);
room_av(406,'double','1st_class',400);


end;
/

drop table customer_info;

create table customer_info(
first_name varchar(20) not null,
last_name varchar(20) not null,
room_no integer not null,
catagori varchar(20) not null,
quality varchar(20) not null,
price integer not null,
date_open date not null,
primary key(room_no)
);

--TRIGGER SECTION
create or replace trigger cust_room
before INSERT ON customer_info
for each row



BEGIN

SELECT catagori, quality, price
INTO :new.catagori, :new.quality, :new.price
FROM room
WHERE room_no=:new.room_no;
delete from room where room_no=:new.room_no;

END cust_room;
/
show errors

everything is working correctly. but i want to show a message like 'The room is not available' if no data found in room table without showing any error. what can i do?? please help

Your frontend (i.e. your application) will need to show that message.

You cannot show a message to an end user from within a trigger.

but in my project i have to show it in backend. i donot have any frontent. if i have any frontent then i can do this. now what can i do to solve the problem??

In the "real world" you would have to do as Shammat says: no user will know that a message was output if it happens only in the backend. However, if this is only an exercise that you will be testing in SQL Plus then you can use DBMS_OUTPUT.PUT_LINE like this:
You will need to issue the following SQL Plus command before running the block to enable the messages:

SET SERVEROUTPUT ON

And just to reiterate: this is not a viable solution for a real system, because in a real system users do not perform inserts by running SQL Plus!

I Reckon this would do what you are looking for

BEGIN

SELECT catagori, quality, price
INTO :new.catagori, :new.quality, :new.price
FROM room
WHERE room_no=:new.room_no;
delete from room where room_no=:new.room_no;
IF SQL%NOTFOUND THEN
dbms_output.put_line('The room is not available');
END IF;
END cust_room;