Compilation error when declaring variables in BEGIN..END procedure PL/SQL - database

Question

I have a compile error that unable to declare variables in procedure PL/SQL which trying solutions from other sources. Can anybody help to check what is the problem?
CREATE OR REPLACE PROCEDURE add_vaccine
(p_id IN VARCHAR, d_id IN VARCHAR, p_vdate IN DATE, p_vaccinated IN VARCHAR)
AS
BEGIN
DECLARE counter NUMBER(MAX);
--Prompt user to input vaccination and visit detail
DBMS_OUTPUT.PUT_LINE('Patient ID = ' || p_id);
DBMS_OUTPUT.PUT_LINE('Doctor ID = ' || d_id);
DBMS_OUTPUT.PUT_LINE('Date = ' || p_vdate);
DBMS_OUTPUT.PUT_LINE('Vaccinated = ' || p_vaccinated);
END;
/
The code is unfinished but I'm stuck at the variable declaration.

Answer 1

Whenever you have a DECLARE section, you must have a BEGIN and END with some code.
If you don't need to declare inside, you can always declare them after AS.
CREATE OR REPLACE PROCEDURE STH()
AS
<DECLARE VARIABLES HERE>
BEGIN
DECLARE
<YOU CAN DECLARE HERE AS WELL>
BEGIN
DBMS_OUTPUT.PUT_LINE('HELLO');
END;
-- Do something here
END;

Answer 2

There is an implict DECLARE before the first BEGIN in your procedure hence
SQL> CREATE OR REPLACE PROCEDURE add_vaccine
2 (p_id IN VARCHAR, d_id IN VARCHAR, p_vdate IN DATE, p_vaccinated IN VARCHAR)
3 AS
4
5 counter NUMBER;
6 BEGIN
7 --Prompt user to input vaccination and visit detail
8 DBMS_OUTPUT.PUT_LINE('Patient ID = ' || p_id);
9 DBMS_OUTPUT.PUT_LINE('Doctor ID = ' || d_id);
10 DBMS_OUTPUT.PUT_LINE('Date = ' || p_vdate);
11 DBMS_OUTPUT.PUT_LINE('Vaccinated = ' || p_vaccinated);
12 END;
13 /
Procedure created.