How do i assign a custom id using a trigger using sequence on a table
You're nearly there! A few small changes needed.

Table definition:

CREATE TABLE Atendant (
   ID VARCHAR(20) PRIMARY KEY,
   NAME  VARCHAR2(25 CHAR) NOT NULL
);


Trigger:

CREATE or replace TRIGGER  Atendant_BI 
  BEFORE INSERT ON  Atendant 
  FOR EACH ROW
DECLARE
BEGIN
  IF :NEW.ID  IS NULL THEN
    :new.id := 'ATN'||Atendant_SEQ.nextval;
  END IF;
END;
/


Test:

SQL&gt; insert into Atendant (NAME) values ('phil');

1 row created.

SQL&gt; select * from Atendant;

ID     NAME
------ ----------
ATN1   phil

SQL&gt;


So all you were really missing was the data type and doing the concatenation correctly.