That was not the issue, useAppMetafields is itself a hook. I successfully got the shop metafield, but I was trying to get the installed app metafield. Shop metafield can be easily manipulated by Metafield Guru app, but the app metafield can only be manipulated by the installed app. I have saved some information about app subscription in app metafield which I want to use in the checkout UI extension for the conditional rendering of the extension.