Recently I used Singapore region to download a game and I changed it back to India region but now I am unable to download amazon app it's asking 18+ confirmation as per Singapore law, it's asking credit card details, but I didn't own one.
As far as I know, India does not require age verification, so something about your description is not consistent with what is actually happening.
Singapore does have age verification but you say you are now using an account set to India. If you are trying to change back to Singapore then you will have to meet their verification requirements. Many countries and regions have introduced this in the past year or so.
According to the Singapore link I provided there are ways other than a credit card whereby you can verify your age:
To download or purchase apps with an age rating of 18+, you need to confirm that you're 18 or older.
You can confirm your age with a credit card that belongs to you, or by scanning your passport, driving licence, National Registration Identity Card, or Foreign Identification Number (FIN) card. Debit cards and gift cards aren't supported.
If you already have an Apple Account, Apple will check if you have a credit card on file or other eligible methods available to confirm that you're 18 or older.
If you choose not to confirm your age, you can continue to use your device — but you can't download apps with an age rating of 18+ until you confirm.
As far as I know, India does not require age verification, so something about your description is not consistent with what is actually happening.
Singapore does have age verification but you say you are now using an account set to India. If you are trying to change back to Singapore then you will have to meet their verification requirements. Many countries and regions have introduced this in the past year or so.
According to the Singapore link I provided there are ways other than a credit card whereby you can verify your age:
To download or purchase apps with an age rating of 18+, you need to confirm that you're 18 or older.
You can confirm your age with a credit card that belongs to you, or by scanning your passport, driving licence, National Registration Identity Card, or Foreign Identification Number (FIN) card. Debit cards and gift cards aren't supported.
If you already have an Apple Account, Apple will check if you have a credit card on file or other eligible methods available to confirm that you're 18 or older.
If you choose not to confirm your age, you can continue to use your device — but you can't download apps with an age rating of 18+ until you confirm.
This is exactly the situation Apple and the app developers are working to disallow ... a person changing regions to try to download an app that is not licensed in their region. When you change regions, you must provide a payment method that works in the new region. You must also comply with the laws of the region. You will not be able to download or use that app in a new region unless you comply. The rules are spelled out in the link below.
Change your Apple Account country or region - Apple Support
Region issue with app download
