My code shows error and I am not able to understand the errror

You have a comma after:

 l_average_cred,

which isn't needed

In fact you have far too much, or wrong punctuation in your code. Here, try this...

CREATE TABLE credit(
  cust_name varchar2(100),
  credit_limit NUMBER(10,3),
  cust_id NUMBER
  );
INSERT INTO credit VALUES('Arnab Deb',999990.000,100);
INSERT INTO credit VALUES('Pampita Bhowmik',999991.000,150);
INSERT INTO credit VALUES('Ankur Kumar Das',999992.000,102);
DECLARE 
  l_credit_lmt credit.credit_limit%TYPE;
  l_average_cred l_credit_lmt%TYPE;
  l_max_cred l_credit_lmt%TYPE;
  l_min_cred l_credit_lmt%TYPE;
BEGIN
  SELECT MIN(credit_limit),
         MAX(credit_limit),
         AVG(credit_limit)
  INTO   l_min_cred,
         l_max_cred,
         l_average_cred
  FROM   credit;
  
  SELECT credit_limit
  INTO   l_credit_lmt
  FROM   credit
  WHERE  cust_id = 100;
  
  DBMS_OUTPUT.PUT_LINE('Minim-Credit:-'|| l_min_cred);
  DBMS_OUTPUT.PUT_LINE('Maximum-Credit:-'|| l_max_cred);
  DBMS_OUTPUT.PUT_LINE('Avg-Credit:-'|| l_average_cred);
  DBMS_OUTPUT.PUT_LINE('Cust-Credit:-'|| l_credit_lmt);
END;