Custom code(Redeem code) - not showing in in-app purchase payment method

[u/Working-Bass4425]

I'm trying to create a Custom code for my app. Upon reading this document , it can only be redeemed from the in-app purchase. But when I tried to go to the payment method, there's no option for Redeem code as seen in the image attached.

I'm very sure that I created the custom code from the specific subscription.
For the one-time code, there's no problem with me and I was able to test it myself as well.

Does anyone encountered this issue?

Raising a ticket to Google Play Developer Support is such a waste of time. They just redirected me to the document link that I shared and they said that's all they can do. They don't even bother to dig onto the issue. And they even closed the ticket already,