It sounds like you are currently under the age of 18, and are seeing an error when trying to add a payment method for your store billing & subscription is that right? Generally you have to be 18 years old to open a Shopify account which is outlined in our Terms of Service: Account Terms here.
Would it be possible for someone like a parent to add a payment method to the account? Something you could consider is transferring store ownership to a guardian such as a parent. You can then add yourself as a staff member and still have access to the store for the day to day running. When you turn 18 you could then transfer the ownership back to yourself. More info on staff accounts here.
In this case I would advise you to reach out to our support team here. That way we can authenticate you and take a closer look at your account to help get this resolved.
If you are brand new to Shopify, be sure to check out our Getting Started Guide for help on starting out.
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