Unpivot with no-constant expression

I have data like this (notice that my month are in format 'yyyymm'):

I would like to unpivot my data using PL/SQL. I have tried the following:

select
    car,
    month,
    amount
from TABLE
unpivot exclude nulls
(
    amount for month in
    (
        amount_1 as 202412,
        amount_2 as (select distinct month_2 from TABLE) /* this doesn't work */
    )
);

In order not to change manually each month, I would like to do something like in amount_2, but I get the following error:

ORA-56901: non-constant expression is not allowed for pivot|unpivot values

Although it is a subquery, it is a constant expression (month_2 can only be 202501).

Is there another way to do something like this? Can somebody please help me? Thank you in advance.

You can unpivot multiple columns like below

UNPIVOT ( (month, amount) 
FOR col IN ( (month_1, amount_1) as 1 ,(month_2, amount_2) as 2 )) 

See an example in forum

Sample Query

SELECT car,month,amount 
FROM 
  
(SELECT * FROM t)
UNPIVOT ( (month, amount) 
FOR col IN ( (month_1, amount_1) as 1 ,(month_2, amount_2) as 2 )) ;

Output

CAR MONTH   AMOUNT
A   202412  100
A   202501  50
B   202412  200
B   202501  20
C   202412  300
C   202501  900

Fiddle Demo

Hope my understanding is correct since expected output is not mentioned.