Column doesn't exist using CASE statement in PosgreSQL

Question

I am trying to calculate the payments for each age range in PostgreSQL.

select
    case
        when age < 50 then "0-50"
        else "50+"
    end as age_range,
    SUM(payment_amount) as sum_payment_amount
from
    (
    select
        age,
        payment_amount
    from
        "002_DM_clients" dc
    left join user_payment_log upl
  on
        dc.client_id = upl.client_id
  ) query1
group by
    age_range;

But I get the following error:

Error executing query:
SQL Error [42703]: ERROR: column "0-50" does not exist

I have no idea what the problem could be.